Well my situation was a bit different because we dated 7 months, and started sleeping together on our second date. We had s*x regularly (every time we hung out), and it was always good. The issue was that in spite of this, she would play me hot and cold, in terms of availability, responsiveness, level of affection, and what she was interested in having with me. After being together that long, really respecting her because she was a rare, hot, successful, smart girl, I was looking for something a bit more serious. She was more interested in FWB plus a lot of validation, which I probably could have been ok with, but her mixed signals in terms of how she treated me like a boyfriend half the time and a f*ck buddy the other half the time is what ultimately scrambled my brains, similar to what you experienced.