Missing him and I shouldn't. What to do?

Well, my story is kind of a long one. I'm 19 years. Been in a relationship for about 3 years. But I feel as though I've made a big mistake. I think I love someone else.

One of my old co-workers and I had a fling when my boyfriend and I split up for a few months. He was nothing but a great guy, constantly putting a smile on my face and making me laugh. He cheered me up whenever I needed it. He really liked me. When he knew I was still talking to my ex it did upset him and he wanted me to choose who I wanted to be with.

At the time, I was not over my ex. I constantly thought of the good times we had. I mean, I spent the past 2 years of my life with him. I couldn't just get rid of him like that. I wanted him back.

So my ex and I got back together. At first I was thrilled, but now I feel as if it's going right back where it went before.

I miss the other guy. But I think I broke his heart by pushing him away and choosing my ex. After my decision we stopped talking, and he now has a new girlfriend who he has been with for almost a year.

I can't really express how much I miss him. But I feel as if he has moved on from me and has no feelings for me anymore.

I'm just so confused on how to get over this. Advice please?