You are right to be worried. Your guy has a serious problem. I can’t tell on the basis of your letter what the problem is exactly, but all the possibilities I can think of are serious and he deserves treatment. It makes me sad when therapists dismiss someone’s pain or suggest that “possession” is the answer. I have to wonder if his mother took him to such therapists because she was afraid of what a more sophisticated therapist might discover.
Regardless, your fiance is now still troubled and turned off to treatment. I have one suggestion for you: Go to a therapist yourself and share what you told me. Make sure you find a therapist you think your fiance will trust. That may take interviewing more than one. Once you find the right one, you and your therapist can talk about how to invite your fiance into some sessions with you. It may be that your willingness to be with him will help him at least consider getting a thorough evaluation.
Do slow down on having a family. You are only 19 and you have plenty of time for your fiance to get well and for your relationship to become solid before introducing children into your life. It’s important that your fiance get a diagnosis — both for his well-being and for making good choices about if and how to be a father. Some disorders are genetic — in which case it’s important to factor that into your thinking about parenthood. But even if that isn’t the case, his emotional “absences” will have a profound effect on both you and your children. Treatment will let him be the partner and father he can be.
I wish you well.