I love Walter The Grump's answer. I would add, getting into a relationship to "fix" you or "them" is severely harming both and those around you. You could do long term damage. Definitely not a healthy idea.
Is it ok for someone with mental health problems to enter into a relationship hoping that it might help them?
I think that's a personal choice, depending who the person is. The best Way to proceed is to really communicate with the person you are entering into the relationship with. The bible tells us that there is a frustrating of plans where there is no confidential talk. Have in mind the things you want to know more about. Most importantly, do your research and be honest. No what you are getting yourself into. Marriage is a serious commitment that you make before God.
Well, love is nature's psychotherapy; but it's not too effective if you seek it for that reason.