This problem keeps following me around...?

My name is Allyson, and I go to a university in Missouri. This isn't a high school problem, I'm a senior in college, but I just can't stop thinking about this. I dated this guy for 2 and a half years and we were definitely in love. However, I decided it was best to split up after multiple arguments with him. We just couldn't see eye to eye on issues that were important to me. A week after we split up, he began a relationship with my BEST FRIEND. I confronted this so called "friend" about her relations with my ex, and she denied everything. Finally, when she'd fessed up, I was furious. She didn't bother asking me what I thought? I would never do this to her. My boyfriend and I were so serious that he bought me a 2 carat diamond ring and we talked about marriage after graduation. We all 3 attend the same college, and I see them on campus everywhere. My problem is, I just cannot get over this. It's been about 7 months since this all started, but I still harbor so much resentment towards them both. Is this normal to feel this way? Or is this an issue I need to seek therapy to fix? Is it wrong of me to be so upset? I just feel so used and taken advantage of by my best friend. Thanks in advance.