I think you know too much about what is going on in their marriage from him oversharing with you. You are not the "free therapist." That might be fueling your jealousy -- him dumping all this stuff on you and you wondering why he puts up with it, does all this stuff to "keep" her, etc.
For your own mental health, and because it's only been 4 mos of dating, I think you could step back and tell him you don't want to know details about their marriage problems, and ask him to find himself a poly counselor for that.
You could also choose to end it with him and tell him to look you back up when he can actually offer you monogamy or a 50-50 split. He also can't offer 50-50 time until 6-12 mos out from now. So you being "on hold" while he gets himself together... I'm not sure I'd sign up for that. Why would you?
It might be great for HIM to keep you around... but I don't see what YOU get out of that deal. Since you are monogamous, putting all your energy into him prevents you from dating other people who might be more compatible and CAN offer you the monogamy you seek NOW.
You could think it out NOW before you invest more here. Would that 50-50 split even be enough for you, to make peace with the fact that you won't ever get monogamy here with him? What if he starts dating more partners?
He didn't want to tell me about things, but when he was upset, I asked him what was going on. I wanted to know stuff. Now he's keeping it to himself more, but it's still hard, as I still know he's upset about stuff. He's not happy and she isn't making him happy.
Who knows what the future will look like for us while everything is still up in the air with her and decisions and stuff. I'm trying to stay positive and hopeful.
I do talk to other people, but haven't met up with or slept with anyone else. If that changes for me, I will tell him.