Every morning I wake up with dread.

You've survived OP - like a shipwreck survivor you're picking through washed up debris trying to create a new environment.

Every day will be a challenge but truly the worst is over. You've made it to safety by your own efforts and will, so take a few moments every day to remind yourself of that.

@SugarPlumpFairyCakes I'm so sorry for what your STBX put you through (and still does). I totally understand how you feel so overwhelmed and it must be infuriating beyond belief to be paying off £22k in unpaid school fees simply because he didn't and won't (I'd want to kill him for this alone!). I've only read your posts so forgive me if someone else has highlighted this, but you wrote in your first post "I wouldn’t mind but these debts are not my doing. But because we were married, I too am liable." This is not true. You are never liable for your spouse's debts incurred in their sole name just because you're married to them. You are only jointly (and often, severally) liable because you were a joint signatory in your own right on the agreement (as you clearly were for your DCs school fees).
Are you currently in debt that you can't afford to service? (Presumably you have some sort of repayment plan for the school fees?). If you're struggling to keep your head above water, may I suggest you contact one of the free debt charities (StepChange, PayPlan, National Debtline and Citizens Advice) for advice - depending on your income and total debt you may be eligible for an IVA or a DRO and may even be able to have some, if not all, of your debt written off. I think if you could get your money worries under control - if not totally solved - you would feel more in control generally (and less like your STBX has the power to re-victimise you every day). Some info: https://www.citizensadvice.org.uk/debt-and-money/help-with-debt/dealing-with-your-debts/check-your-debt-options/
Your 13yr old DS may want to go and live with your STBX, but how does your STBX feel about this? Have you discussed the possibility with him? (He may say No outright, which would solve the problem as it would no longer be you telling your DS that he can't, but his father refusing to have him.)
Finally, I don't think you say in your posts how long you've been in your current job. If it's more than two years you may want to think carefully about leaving it for a new job as with more than two years' continuous service you have full employment rights e.g. protection from unfair dismissal not involving discrimination (which is a Day One right) and redundancy pay, but with fewer than this you don't (this two-year rule is due to be replaced by a shorter statutory probation period as part of the new Employment Rights Bill, but not for another year or two).
I think, considering you're taking care of four DC and holding down a job, you have more resilience than you give yourself credit for, but very best of luck with however you choose to move forward.

this sounds horrible and I can absolutely see how it feels like a groundhog day scenario.

For me - I have to do practical things - so you have three key worries and two of them at least you can probably take positive steps ib

1. Job - I wouldn't try and do a big change like cybersecurity training at the moment if I were you - yes there are things you can train for that earn good salaries but there's no guarantee you'll have an aptitude for them and no guarantee in the jobs market at the moment so I would avoid specialising too much. Your current employer sounds horrible so I would focus on trying to find a nicer version of what you currently do initially, if possible. Then you can be confident that you can do the role and will have one area of your life where you are happy and stress free for 40 hours a week. There are a lot of free online courses at https://freecoursesinengland.co.uk/free-courses-1/ which will make you more confident in what you do and more desirable as an employee - and to help stop the immediate criticism of your canva skills there are also courses either in using Canva or in basic design principals (check coursera) . No you shouldn't have to but if it gives you confidence to tell them to feck off then why not?
2. Money - it sounds like you only owe the school and have already done the right things and talked to them - could you go back and say you absolutely appreciate their help but even the current payment is causing issues, can it be reduced again with a firm date to review with a view to reinstating higher payments? Ask for a year on lower. The reality is if they refuse and you stop paying entirely they'll be worse off so they should agree
3. Teenagers - harder - can you ask your older children to help, get them to choose a film he would love to watch and all have a movie night which he can join or not join as he chooses? Family tickets to somewhere he would actually like to go? To be honest when I was 13 I didn't want to do anything with my parents either, and there's nothing you can directly do to compete with the (presumed) promise of no rules and chips for dinner every night at his dads, so best advice may be don't take it to heart too much and just keep being his mum, hard as that sounds right now.

Then his dad screams in his face without you being there. If it’s bad enough, he asks if he can move back to you. Which is fine but he won’t be in his old school and will likely be in a not so good other one. A man screaming at a teen isn’t ideal but it’s also not the worst example of parenting I’ve heard of and in a year or so, your DS will likely match him for size.

This.

If your son is making your life a misery and things his dad is amazing, he needs to go and live there and find out the reality for himself.

Then his dad screams in his face without you being there. If it’s bad enough, he asks if he can move back to you. Which is fine but he won’t be in his old school and will likely be in a not so good other one. A man screaming at a teen isn’t ideal but it’s also not the worst example of parenting I’ve heard of and in a year or so, your DS will likely match him for size.

Just make sure you always leave the door open for DS.

Perhaps suggest to DS to do 50/50 now and if he still wants to live there more then you can arrange it after Xmas.

DS is struggling and you have too much going on right now to add this to it.

Do you have other kids?

How much do you get paid in your current role?

How long have you been there and have you spoken to them about doing things that aren’t in your job description?

Not much to add that hasn't already been said. If it were me I would let 13yr old trial staying with his dad (maybe at weekends and a couple of weeks over school hols?). Your understandable misgivings are falling on deaf ears and he won't get it until he's experienced the reality for himself. Tell him he will always have a home with you and can come back anytime if it doesn't work out.

Keep your head up and keep going, the ex is gone and you can get through this.

Re the debts - have you phoned Stepchange? It might be that entering into an IVA is the answer for you. And I think Surviving Economic Abuse can also give you support too.

I sympathise on the feelings of dread. I've been divorced 10 years and I still have nightmares about my exH. I still can't hear his voice. Have you tried counselling? It has helped me a lot.

On the job front, have you spoken to your line manager about what they're asking you to do? Or HR? Can they provide you with some training?

And in relation to your 13 yo - I think you probably have to let him go.

Ah okay. Well I would have a very serious chat with your DS and tell him he can go, next school year but that this means if he ever changes his mind, his current school place will be lost. Be brutally honest about it with him that you don’t think he will like it. If he insists then he has to live with the consequences if he changes his mind. By then he will be 14 so I think that is fair enough. If your ex went to court about it, the court would allow the child’s views to be decisive most of the time at this age.

Agree with others to allow DS a trial period living with dad. And could you move the kids to free education?

Get your 13 year old to his father but with the caveat that whatever comes his way is his choice. If he kicks off, tell him to take responsibility for his choices and he has to live with them.

I get you want to protect him but it is likely to end up backfiring and the outcome could be even worse. How about he goes to his dad at weekends and holidays to allow him to keep the school place?
I presume the kids are all out of private school now because if you’re in debt they really should be.
What does your solicitor advise you’d be able to achieve by way of a divorce settlement?

I honestly would consider shared care for the 13yo. There's absolutely nothing you can do to change how he's feeling other than letting him experience life with dad for himself. I've got 3, all young adults now & all had very different opinions about, & relationships with, their father growing up. I stayed completely neutral & there was nothing I could have done to influence them anyway. If you just keep saying no to him he'll resent you, & that's THE most destructive emotion in my experience.

Re career. I went back to uni in my 40s & retrained into the professions. I have to work temporary contracts to maximise my earnings at the moment, so it's hard going with no holidays or sick pay, but it's guaranteed work, age is valued & we live well on it. It's a descision I've never regretted & a very successful investment of time, effort & money.

Take your dc out of private school and declare bankruptcy if eligible. Start afresh.

Ah sorry, x post. Can see you’ve already spoken with school,
you are doing all the right things, do you hzz as be someone IRL to talk to?

Have you spoken to the school about the fees? When we found ourselves on verge of bankruptcy with our former business I was very honest with school and told them we couldn’t keep a roof over our heads let alone pay fees.
they were surprisingly understanding and agreed a long term interest free payment plan that will take another 4 years to clear (well after DD has left) but has meant I can just about keep my head above water. I pay half, ExH the other.

Have you spoken to woman’s aid. They have a course for teenagers. They can also put you in contact with companies to reduce the debt as it is not yours. Is your solicitor in family law? You need someone who understands. No way you should be paying those debts if he sold the house.