Despite his immaturity, is he at all supportive?

I know this sounds 'hokey' but I think you need to sit down with a pen and paper and draw out a few lists. Perhaps EVIDENCE FOR Vs EVIDENCE AGAINST the relationship being workable. And of course a list about ADVANTAGES TO LEAVING Vs ADVANTAGES TO NOT LEAVING.

I've been using such strategies for a while now. They not only help me in my decision making (specifically that I don't act rashly) but give me comfort that I have chosen what's best for me.

Again, it sounds absolutely silly; but, you have nothing to lose by trying it.