Thinking about the past..

Why can't I stop thinking about the past and what I had before? I'm sure I'm not even a thought for my old friends anymore, but I can't seem to let it go. I always felt like a therapist to them though. I don't even know if they ever saw me as a friend or a therapist the whole time they were my "friend". I felt easily replaceable with them too. I was in a relationship with them at one point but they went for another one of my friends as soon as I introduced them to eachother. My friend that they went for ended up unfriending me for no reason and would say I did nothing wrong when I tried talking to them.

They did eventually break up though and my friend came to ME to vent and didn't even ask. But when I told them something simple like "I'm gonna take a break" they could care less? All they said was "ok?" Like... wow.

The amount of times I've waited on them, the amount of chances I gave them, and they never seemed to care. I confronted them too about the "relationship" between us on an alt account when they randomly blocked me one day, and they said it happened when they were in a bad mental state. Idk anymore.

I miss them, but at the same time I never want to see them again. And I have decent support now, but I just can't get it out of my mind. What do I do atp?