Arguing

Me and my girlfriend argue about sex a lot. I wish we wouldn’t but I just get upset because I don’t see her a lot so the time just builds me up and I wanna do things when we’re together alone, but she hasn’t been in the mood at all, yet when we first got together it was the other way around. How can we fix this, every time I try to talk about it she either ignores it or changes the subject so it makes me mad then we argue.