Fear of abandonment is a b"tch. How do you guys deal with it ?

I have a sort of a relationship with a guy. This guy was into me for YEARS. I got married and said goodbye to him but he kept trying to contact me. Blocked him from everywhere. Now that the marriage fell apart I finally replied to him. and it was going soooo well until 2 days ago I sensed some shift in his tone ??? And it gave me a weird fear. Then since yesterday he seems angry at me for something (slept with the divorcing ex husband just for a night .. he begged me and I gave in). And he says he isn't angry and understands my position but I don't know. I feel like he is leaving me. This fear is overcoming all the rational evidence including the fact that he has been crazy about me for years. I started talking to other guys today to distract myself and to protect myself in case he LEAVES me. Help me out in this please 😭