To hire a private investigator to follow my ex?

I completely understand why you want to know. I would too. But looking at it objectively what does it actually change? Whatever his reason for leaving his treatment of you is abhorrent. As pp have suggested the best response here would be to tear off the plaster and cut him off. No more staying over, no more long cosy chats. He’s not your husband and he’s been a twat.

I’m not going to tell you not to get the PI, that’s your choice. But it will be far more valuable for you to have the courage of your convictions and cut this knobber off properly. He’s been a complete dickhead and even though you’ve divorced him he essentially still has a wife of convenience. The only thing you need to speak about is the kids. Don’t let him keep using you.

I agree with pp, this sounds horrendous and I’m sorry you’re going through it.
In your situation I too would absolutely want to know what’s going on
But as pp said - whether or not there is another woman doesn’t change your decision. You (entirely reasonably) don’t want to pick up the load for everything for two months (although realistically, this would be indefinitely) while he weighs up his options. So you are leaving him.
I also agree with others saying you need a cleaner break without the drawn out phone calls and weekend visits; it’s causing you more pain.

OP,

this could have been my post. Staying with relatives and staying at mine at the weekend. Putting on a show for the DC.
spoiler, after a year of this stupidity, his mistress got fed up and wrote me a letter telling me he was playing is both.
I got rid of him and 7 years later, he’s not speaking to me because I filed for divorce eventually.
You don’t get to ‘find yourself’ when you have four dc 🤦‍♂️
Be strong, my family live 3000 miles away but I’ve done it myself.
you can do this x

So what will happen if the investigator tells you there is no other person involved and, in fact, he is having regular appointments with a psychiatrist, counsellor? What would you do then?
You know why he left because he told you. That should surely be enough for whatever closure you are seeking, whether you think his reasoning is sensible or not.
As far as him finishing his masters, I did it while being medicated for depression and anxiety. My friend did hers while recovering from a drug dependence. Being able to complete education is not a sign of thriving mental health. In fact, I moved out as well when I was in the final year of my masters because I couldn't handle the pressure of living with my partner at the same time as dealing with intense workload and my ill mental health. Admittedly, we did not have children. Similarly to you, he also decided that it would be better to separate permanently.

Ask for his medical notes

To be honest, I don’t think it matters if he has another women. He has treated you appallingly and you owe it to yourself to cut him off now

it sounds to me more like a midlife crisis than another women though - he wants to try life without a family but keep you waiting in the background as a contingency

One of the best bits of wisdom I've ever heard is "closure is a gift you give yourself". It's not something that comes from outside of yourself.

You can never truly know or understand another person's motivations, even if you know the facts. They may not really understand their behaviour themselves - this is often the case!

You can give yourself agency. Work with the facts of your life. Work towards the life you want in a way that doesn't involve puzzling over someone else, and which gives you and everyone clarity. I don't know why you are still in such deep conversation and contact with your ex given you choose to divorce him. Time to cut the cord as much as possible, bearing in mind you are co-parents.

The difference between a relationship and a non-relationship isn't sex. Lots of people are in sexless relationships, and have sex with people they aren't in relationships with. The difference is emotional codependency. You need to break that off with your ex or you'll never truly move on.

Don't hire a PI. Give yourself closure. It doesn't matter if there's another woman or if there's a different reason. The result is the same. So decide it's over. Decide to believe it's over. And act like it is. That is your route to closure.

There may not be anyone now. But imo it sounds like there may have been at one time.

He says he still loves you and hopes to get back together some day. If this is true, how long are you willing to wait for this person who didn’t love and respect you enough to stay and work on the marriage (or even do his share of parenting). How much more of your life are you willing to waste on this man?

Perhaps your ex was actually telling you the truth - he DID have a mental illness and could only deal with it by being on his own. If that is true then your response, to threaten him with divorce if he went away, was awful.
Losing weight is a sign of being mentally unwell. Coming home from work on time every day and spending every weekend with your family is NOT a sign of someone who’s having an affair.
You may have treated your unwell husband appallingly! Please don’t continue to do so by paying someone to stalk him.

How would hiring an investigator give your marriage back, if my husband did this to me no matter how unpopular saying this on here is I would think he was unhinged

Closure doesn’t happen with an open door.

I agree with PPs, you want to know what he was/is thinking so you don’t get used

But you are getting used by continuing to have contact without enforcing this boundary

If you’ve told him you want to know and/or move on and a divorce would suggest you have! but continue to behave as if still very close then of course you won’t be moving on any quicker than he’s going to spill his reasons

I imagine he knows the explanation of “I have all the support and attention I want from you and now the ability to keep more secrets, shag about, and avoid any family domestic responsibilities including child-rearing and day to day care” probably won’t go down well & his cosy set up will be buggered up

Tell him to shit or get off the pot.

He’s putting you & your children through abject hell because he’s a selfish entitled prick only thinking of himself. Acute mental health problem & doing a masters? Yeh right. He just wants an easy life where he doesn’t have to parent his 4 kids and yeh, I’m sorry to say this but chances are there’s most likely a woman. Hardly any man ever just randomly left and stayed on his own- they generally can’t cope on their own. I’d say you owe it to yourself & the kids to know exactly what he’s up to. His treatment of you all the secretiveness and breadcrumbing is horrific. I couldn’t forgive this. Do it for your own answers cos he isn’t going to give you any….and don’t you dare feel an ounce of guilt about it! X

Well, it's a bit late now to call the PI. He may not have had another woman/man in his life when he left - but after 18 months he most probably has.

He's using you.
He's overwhelmed by family life so wanted that break. But occasionally when he's a bit bored, nothing going on, he fancies seeing the kids in an easy location he rings you for a chat or comes over. And then he leaves.
My ex does this. And I cling to those small moments but I ate myself for it. So be stronger and put a stop to it

Yes an I would agree try counselling first.

To be honest it's his own business at this stage if he has a new partner. You are no longer together.
Draw a line under it. Why are you still talking to him and having him stay over? He probably thinks he can try and keep the bits of your relationship he liked, while also having his entirely new life. You do not need to spend money having him followed.

I agree with the pp that confirming there is another woman will not necessarily bring ‘closure’ and you have to be prepared for that. He will probably continue to gaslight you in one way or another. He is not treating you well either way. But if it helps you to know and you can afford it then go for it.

Don't use your kids as a way to justify getting a PI. I actually think if you want or need to know then that is your right.

Whether then you have new things to deal with is down on you. Well done for sticking to clear lines with him - that's the best thing you could have done.

Regardless, use some of the money for counselling. It will be worth every penny.

He dumped all the hurt on you and left you to care for 4 kids while he looked after himself. Regardless of if he's got another woman or not can you forgive this? Mental health struggles or not is it fair to walk out on your children and leave their care solely to a now over stretched mother? Is that forgivable? Maybe he had another woman or maybe he just got fed up with domestic duties. Now he gets to have the long conversations and all the special attention from you with minimal effort from him. He's got the best of everything.