When friendships go bad, how do you let go?

This girl was a friend of mine for a long time but... well, let’s say things didn’t just go downhill, they fell off a cliff. We haven’t spoken in a couple of months. Logically I know trying to continue the relationship isn’t worth it because the bad outweighed the good, but emotionally I keep remembering the good times and being sad. I’ve lost friends before, but this seems worse than the other times. I know time will help, but any thoughts/advice for right now?