Sex Drive

My boyfriend and I have been in a relationship for 3.5 years. He is 22 I'm 19. He always wants to have sex but I have no drive...it's not fun it doesn't feel good or bed. I get super nervous and anxious when we are about to do it...so nervous I stop him and we end up not having sex at all. It's a mixture of nerves and just no drive or need for it...it's starting to affect our relationship in a negative way. Please help me find some guidance on what is wrong with me...