If anyone has any tips about this I’d be grateful. Any books or experiences that helped get over this?

I was with a girl over 6 years ago (for only a couple of months) she asked me to marry her and I said no. I still regret this. She finished it and does not want any contact. I’ve had a long term relationship since but I still think about the girl of 6 years ago.

Any suggestions would be gratefully received