What should i do with this girl?

So i started hanging out with this girl about two years ago we never did anything just spent a lot of time together... the kicker... she has a boyfriend of 2 years... So after about a year of hanging out she knew i liked her but we went through a period where she pretty much told me off and we didn't talk for about 4-5 months. She tried to start hanging out with me again but i was to pissed at her to give her another shot. She continued to call and txt me and i wouldn't respond. Eventually one night she called me and i picked up we talked and we started hanging out again.... She eventually started sleeping over at my place and then we eventually kissed and then it went even further... We started having sex a lot Things were really good for a long time like really really good i felt like she truly cared about me and that she wanted to always be with me.... But now were going through another spell where i feel like she is pushing me away. I constantly take her out to dinner and buy her really nice things because i like giving and i truly really really like this beautiful girl. But like i said just recently it feels like she is pushing me away.. we have stopped talking again and i don't know what to do. It might be because she is back home with her boyfriend.. Any advice is greatly appreciated please don't beat me up on the topic im just looking for advice to help me.