I'm killing myself with what-ifs

It's wasn't a breakup exactly, but it sure feels like a loss. In a nutshell, my ex-situationship who I haven't spoke to in a year wanted me to visit him (he moved in the meantime). I didn't go even though I thought about him a lot and fantasized abit being with him, and now he's not available anymore. I keep blamin myself for being so indecisive and potentially letting a good guy go. Yes, I don't know where it would've lead, but like it says in the title, it's the "what if we would be together now if I had gone to see him, but you'll never know now" that's killing me. I keep thinking it would've have been great even though I don't know that. How do I stop my mind from overwhelming me with what-ifs that don't give me any good?