Ex boyfriend had sex so soon with someone else?

We was together for 5 almost 6 years before we broke up. I have the feeling he was having disconnection with relationship thus why it seems he's moving on faster than me. It's been going on 2 months since the break up. He was still talking to me and helping me pay some bills as we lived together and his name was still on lease even though he moved out. He has a female co worker I always knew liked him and got that I didn't like her. She also slept with my ex boyfriend best friend as revenge against her cheating ex boyfriend. Now two months later her and my ex hang out all the time and I just found out the went on a day trip somewhere stayed at a hotel and had sex. It took a lot for him to admit but he finally did but got upset that I knew so much. Called me crazy, stalker (I'm not his iCloud still on mac so his booking info came) told me he's officially done with me not helping me with bills , doesn't wanna speak or see me ever again. I know it sound like girl be done with him but I can't get the idea of him with someone else out my mind as we were each other's first. Then I keep thinking what if she gets pregnant what if starts falling for her. I lost so much weight since the breakup (40lbs) missed work because I'm so numb and feel so empty. He tells me to leave him alone move on and he has officially blocked my number. Everybody keep telling me to let go but I don't know how to?
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Update... he blocked my number however I guess sometimes unblockes it because on Tuesday I wrote him a letter to express how I felt since he said he didn't want to speak to me. Basically saying all the stuff we went through, not understanding how he could treat me the way he is... never mentioned girl situation but told him he truly hurting me and will one day regret itA co worker gave it to him and about a hour later I gets a text saying" I got ur letter, honestly I just want to be left alone
Ex boyfriend had sex so soon with someone else?
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