I know what you're talking about man, my girlfriend now was my highschool sweetheart and all that bs but before we started dating again, from about 15 - 20 her and I kept going back and forth. I fucked her over she fucked me over, we dated other people etc. It may seem like the worst thing in the world and it's definitely depressing as fuck but I promise that dealing with it will make you stronger. And you NEVER know what life has in store for you, at one point I was dating another chick and she was dating another dude and I would still think about her every day and about how I will never be with her again(because I was sick of going through all the bullshit) and before I knew it, her and I were brought back together somehow and were both happier than ever. Been dating the last 2 and a half years.
Long story short, just take it one day at a time. And there's only so much that you have control of.
