Wow that was one frustrating thread.
I had come to give you advice on how to find cheap holidays for you and your family, but turns out you actually have the means to go on holidays but just doesn't want to spend the money. That's fair but the whole blaming it on your ex is a bit... 
Yes, your ex sounds like an arse and you obviously still suffer from PTSD but you really need to snap out of it and get a grip, like really.
Plenty of women are abused and left with nothing, and I mean litterally nothing (and definitely not any kind of money in the bank), I have met a lady in Mexico a few months ago who haven't seen her children in years because her abusive ex-husband had her deported of the US an got full-custody, her youngest was a baby at the time and is now 5. My mother was in an abusive relationship with my father, he brought her to court countless times and didn't pay a penny despite never seeing us. My mother was mad but she also understood that it's the risk you take when you have children with the wrong kind of people. She also has 50:50 Custody of my youngest siblings with my step-dad, she understands that it's a risk you take when you have children, because the relationship might break off and there is often no reason why one should have more custody than the other, and no, she isn't entitled to CM from him because he has them 50% of the time and therefore already pays his share.
You already got money out of your ex, money that you are choosing to keep for a deposit which is fine but he doesn't owe you maintenance, because you wish you could have more money, what he earns is irrelevant because you already said he pays things for his children, including holidays therefore there is absolutely zero reason why he should pay for your holidays, the same way I doubt you would be keen to pay for his holidays if you earned more than him.
You need to acknowledge that while you didn't decide to be in an abusive relationship and lose half the custody of your children you are 100% responsible for the choices you make.
Again you didn't chose to be in an abusive relationship, but you made a choice to have children and it so happen that you had them with someone who is abusive. It is unfortunate but you are unfortunately not the first nor the last woman that happens to, and while having 50:50 custody might hurt, you might want to acknowledge that you are partly lucky to have an ex who seems to be pretty hand on and happy to spend money on them (booking therapy sessions for his anxious DS without prompting, taking them on holidays etc) and actually keen to have them for half the time as plenty of dads would have deserted the second the marriage broke down. So while you are not unreasonable to be sad about the situation, you might also want to acknowledge that you probably don't have it as hard as plenty of other mums in your situation who have dads that pay zero maintenance, despite mummy having them 24/7 and all the costs that that involves.
It sucks that your job brought you in a bad place, but again it is your choice to stick to a job you love but pay pennies. Is it infuriating to see your ex earning 6 figures and living the big life while you struggle? Maybe, but you are choosing to live in poverty by privileging more time with your kids for less pay instead of a higher paying job that would mean a better quality of life. Again, it's fine, but being broke is a choice you are actively making in your case and doesn't seem to want to fix because you somehow feel entitled to your ex boyfriend's money.
You have money in the bank you chose not to spend, again it's fine but you don't get to moan about being on the breadline when you really aren't and could easily afford an holiday if it wasn't for you wanting your own house instead.
And no you are not the only one facing renting long-term, your children and most of the current younger generation will never have the means to buy and will be stuck renting long-term and plenty of people are facing a life of renting because they can't even put a penny aside each month, let alone a house deposit.
It's fine to moan about the bad things in your life, OP, but it's always good to take a step back and look at the positives. In my book you would be seen as pretty privileged in many ways to be able to make the choices you do and still have a house and food on your table and clothes on your children's back and even money in the bank.