Wanting to be taken care of and wanting your girlfriend to take care of you are really poor reasons to get married. In fact, they will probably doom the marriage from the start. It is really unfair to expect someone else to take care of you all the time. Caretaking of a spouse is a HUGE strain and burden. Sometimes it is absolutely necessary and unavoidable, but you should be able to learn to manage your own diabetes, and if your mental health issues are beyond your ability to manage, asking someone else to manage them for you just really isn't realistic or really terribly fair.

Sounds like you really need the group home. Go there with the intention of learning the skills to manage your own health and well-being. Learn to do that and then perhaps down the road you can consider a marriage of equal partnership.