Why am I so attached after he did that..

I’ve been with my now ex bf for 5 years & a few months ago I found out he had an old man suck him off for $300... I don’t believe that was the only thing that happened.. but that’s all he claims happened. I stopped having sex with him & ended up finding out he was being sneaky. I had him leave our apartment but now I’m constantly concerned what he’s doing, what girl he’s with & it’s like I have to remind myself why tf am I worried about a man that cheated on me with an OLD MEN...????? Possibly could’ve given me an STD?? But there’s still times I blow his phone up just sad about it. He was my person.. I don’t really have family & maybe 1 friend.. we lived together for the last 3 years and idk just miss him around and bothered by the thought of him being with someone else