Sexually incompatible and feeling trapped

I've been with my boyfriend for almost three years, we got together in the first month of uni and have had a very intense relationship, having the same group of friends and living together in our first year. Over the last couple years he has been bringing to light the fact our libidos are so mismatched. He would like to have sex at least 3 times a week whereas I only once a week. It is an issue he keeps bringing up because he is feeling so frustrated and is a topic that makes me incredibly emotional. He claims I just say I'm sorry and don't try to do anything to fix it but I guess I get so upset because I know how much it means to him and I really really struggle 'feeling it' anymore than I already do and I really don't want to force myself to do it when I don't want to. Recently we have been having sex about 4 times a month and doing other stuff about once a week but he feels that I never initiate anything and he has to ask or imply. I feel like I have been trying to a bit more but obviously not enough for him. I feel terrible that I can't give him what he wants and I really don't want to break up but it is always in the back of my head and I feel so trapped as to how I can make it better. He also claims I don't give him enough physical attention, not just sexually but I have always been the 'colder' one in the relationship whereas he is quite needy. I really don't know what to do. I'd love to be able to give him everything but I physically can't make myself want to have sex more. Please help!

"and I really don't want to force myself to do it when I don't want to." Oh? Why not?