What you have to consider is, this set up is never going to change.
His ex will always be there, and so will his kids.
His mental health issues will always have to be managed, they don't simply go away.

I don't think your asking too much, but I think your asking for too much from him. If you get what I mean?

He can't give you support, or promise you stability. He may well be terrified of having anymore kids in case the whole nightmare repeats itself.

To want these things is perfectly reasonable, but to want them from him...It just can't be done. Not for the foreseeable future.

If you really want to be with him, you need to support yourself, provide yourself with your own network and be available when he is present and able.
You may also need to understand that you won't take that first place position while he is fighting for his family.

This isn't personal in anyway, I have no doubt he wishes things were different. And that's what makes this so sad.

You probably do care about each other dearly, yet things have conspired against you both.
I hope you find your answer. All the best for the future, whatever happens.