I'll have sex sometimes even when I'm not in the mood and I always feel terrible after. Like that was a wasted sexual experience because I wasn't fully present there with my partner. Sex in a relationship is so important and when you're with someone for a long time you sometimes forget how that person once made you feel. Until you feel it all over again. Then it reminds you of why you fell so hard in first place. I guess what I'm saying is that if you're (I'm) not in the mood for sex, don't do it. You're supposed to let yourself go during sex, and actually desire it. Cuddle with your partner instead or watch a movie together, save the  good stuff for a time when your bodies are both in sync.