Your H sounds like a codependent. He gels with women he feels are below him so he can fix them and be their hero.
I've been thinking lately that maybe he feels intimidated by the fact you will be a doctor soon. Like he doesn't measure up as a man.
I know for me, I take ALOT of pride in being the breadwinner. I'm a provider like a lot of men are. It defines who I am as a man.
I knew one guy whose wife made a lot more money than he did. You what he did? He worked 3 jobs day and night JUST so he could say he was the breadwinner. That's how important it was to him.
I'm not sure if your H is like this or not but from what you said about his previous GF's, there appears to be a pattern.
Think about it.. why in the ever loving hell would a man leave a soon to be doctor for someone unemployed with psych problems???