Relationship regret

It's gonna be a year since me and my ex broke up cause I didn't act properly and didn't treat him that good acoording to him so he gave up and just broke up after listening to someone else. I was hurt and realised I liked him a lot so I asked him out within a few weeks but he rejected me. We don't even talk that much as friends anymore and is it weird that I still care and he doesn't? I wish I didn't make that mistake, we would have still lasted. How to get over regret..