How do I move on?

Hello- I am in my mid-twenties, have been with my boyfriend since I was sixteen. We have a daughter together and we live together. I have been working at a company for about a year now, a couple of months after I started working there my boss brought in his brother to work there. The brother was always nice and friendly but I never saw him in any other way but a coworker.

A couple of months after he started working there my boss decided to take a couple of us out for dinner and drinks. Everyone got way too drunk and my boss brother and I ended up kissing, a lot. The next day at work was a bit awkward but we got through it and actually started talking a lot. I asked him to send me a picture we took to get his phone number. We started talking all the time, hang out together outside of work both with other coworkers and alone, and sleeping together.

He explained to me from the beggining that he wasn't interested in having a girlfriend because he has been recovering from some drug issues he had and he feels he needs to focus on himself. I was fine with that because I obviously had a boyfriend already. After a couple of weeks of having this affair I talked to my boyfriend and just told him I wasn't happy and we took a break from our relationship and I kept talking and sleeping with my coworker.

About three months later my boyfriend went through my phone and found out everything, even saw pictures. He was very upset and hurt. I told my coworker what happened but the selfish person in me told him I still wanted to keep talking and sleeping with him. After this happened my coworker began distancing himself from me, he stopped texting me, kept trying to avoid hanging out with me and even began ignoring my calls and texts.

My coworker and I finally texted a couple of weeks ago and he said we should just be friends, I agreed. It's so hard for me at work, I feel so awkward when I'm around him, everyone at work knows what happened between us. He told every one including my boss and even shared some private pictures I had sent him with my boss and other people. How do I move on? Why is this guy still on my mind? I get so jealous when he's taking to other coworkers, going on smoke breaks with them or if I hear he's going somewhere.