Why did he quit his job? How can he afford a truck if he quit his job?
What are your ages? Does he have children with either of you?
Any marital problems before this happened?
Has he ever cheated?
Thank you for that additional info. I'm sorry you're going through this now at this point in your marriage. The first thing I would question is if his medication is really doing what it needs to do to keep him balanced. He's obviously made a few bad decisions recently and it may be because of that.
Aside from that, the problem is when someone else makes a decision, there's really nothing we can do to change it, they have to be the ones to make up their mind. The only thing you can really do is talk to him, about the fact that she cheated on him and you never have, that she dumped him and you have been there for him, that you've supported him and been there for him through his illness, through his job issues, through everything and you are still there for him.
Unfortunately, if he decides to go back with her, there's not too much you can do about it, except try and be strong for yourself, and know that this is not your fault, this is a decision that he's making on his own and that you tried the best you could to make things work out. I welcome your thoughts, let me know if you want to talk more