How do I move on from a fallen out friendship?

Everyone involved is late 20s.

I had a close friend I thought would be there for life, but it wasn't the case. Although we still talk and are cordial, I still have a lot of resentment and hurt.

Sometimes I get better and forget them at all, but once in a while I start looking back again searching for what I could've done differently or if there is still a way to save the friendship (I wish there was, but there is not). I feel like I spent everyone's ears on it by now from how much I talked about it.

When will I be able to let it go? How do I deal with my feelings of hurt and nostalgia for the good times in a health way? I feel so betrayed because I considered them family, it was a huge deal for me.