Please understand that I do understand how painful this is for you both. Pain is pain regardless of whether it makes any sense. But I do have to tell you that your family may be right. What you are describing has all the hallmarks of a codependent and potentially dangerous relationship. Neither of you can function without the other. He accuses you of cheating when you are just living your life and, worse, he subjects you to emotional blackmail by threatening suicide if you’re not at his beck and call. On your end, you can’t function without him. Not good.
Strong relationships that last are relationships between two strong, independent people who want to be together because they love, trust, respect, and enjoy each other. As romantic as the movies can make it sound, needing another person desperately is just that — desperate. it’s not healthy. It limits both people instead of helping them grow.
If this guy was right for you, he’d be encouraging you to do well in school, to go out and do things that are part of the college experience, to have friends, and to be happy. He’d be your biggest fan in your efforts to grow as an individual. If you were right for him, you’d be telling him to take his many problems and suicidal feelings to a professional therapist instead of relying on your untrained efforts. You’d be encouraging him to get off the computer and to take charge of his illness and his life. You’d be his biggest fan in his efforts to man-up and make an adult life for himself. If instead, you two repeat your life of being joined at the hip and retreating from the world, I predict that neither of you will become a fully functioning adult and your relationship will eventually end badly.
As painful as a breakup might be, I still think it might be for the best. On the other hand, I’m judging the situation only on the basis of a letter. It would be a good idea for you to seek out a therapist who can hear the whole story.
I do wish you well.