Why dosnt he want as much as i do?

i am 27, and attractive. i have been with my bf for 2 yrs. for the past 3 months or so, he only wants to do it like once a month. he says he's tired and has add and his meds make him not want to. but he has been on them for a year and it didn't used to be like this we used to all the time. we have talked about this prob alot, but nothing is changing! is a stress reliever for me and i need it more than this. have any of you experienced a lull in this area of a relationship? and what did you do about it?