Well, if he won't tell you what's wrong, and if he won't see a doctor (in case the cause is a physical one), it sounds like you are going to have to learn to live in a sexless relationship if you want to be in a relationship with him. Don't take it personally (you say "it's not doing much for my self esteem") because it doesn't sound personal. I.e., it pretty much sounds like he doesn't want to have sex with anyone, not specifically doesn't want it with you.) I think there are women in the world who would happily live in a sexless marriage, maybe he should be allowed to find one. It sounds like you aren't one of those women, and you've had plenty of warning of how things are going to be if you used to have to twist his arm to have sex as long as a year ago. Why not ask him to buy out your interest in the house, and head on and find someone who likes having sex and makes you feel all boosted up and desirable? If you're only 21, and you don't want your sex life to be over, it doesn't sound like you have much choice.
Sounds like ED, but it takes a women to tell us, which can be a good thing or a bad thing, depending on how your man fills, you could get him some Tribulus, its a herb that will boost his libido, testostrone and his energy, with get him on some Cayenne and some Garlic these last to will help his blood, you can google these with ED on the end, but please tell him you have been looking for help for him, it may help.