is it more an emotional or physical issue? if its physical, i'd suggest seeing a doctor. if its emotional, i'd guess it has to do with hormones. either way, the most crucial element is how you feel about your relationship: do you really want it to work out and why. that's something you have to answer for yourself. if you do, than i would suggest making an effort to work through this issue. if i had to guess, i would say that some of his "disappointment" is actually concern that you've lost interest in him. if that's the case, than making an effort to solve the issue should be reassuring to him. in the end though, i guess it just depends on how much you care.