Strong bump here but OP, did he spend most of his time with you and at some point made you the center of his universe? From what I could tell, he did.
This my friends was a result of a clear lack of sexual polarity, often because the guy makes the mistake of putting the girl first instead of his own mission. Women hate that. Sure the sex may have been great at first but if the guy makes the girl #1 and always wants to spend his time with her, who cares if the sex was good or bad, she's not going to feel the polarity and in turn she won't feel like having sex with him and when they do have sex she'll think that it's terrible/bad/etc.!!!
What happened here with OP was that her former boyfriend probably spent too much time with her like a wussbag as the relationship went on instead of focusing on himself which led to her not feeling like having sex with him. So guys, if your girlfriend keeps telling you to put her first don't do that otherwise she'll go f*** other people who don't put her first!!! In this case it's not about improving the quality of sex, it's always about her feelings and how she felt.
Good luck in finding someone that makes you happy.