I want my ex back after 4 years

5 years ago I dated my neighbor we were both in HS. I just moved back into my parents after being gone for 3 years. My exs parents officially have moved out of their old house. It's weird but kinda nice as after we broke up being so close was terrible. My ex is still in college and has about another year till graduation. I'm working on getting over him as I've been in rebound a lot since our breakup. I wish they were hope for us in the future. He has a gf right now and when I moved out of state which he knew of, she's actually from that state. They go to school for basically the same thing and she's graduating this year. Yes I've FB stalked her page, which was on accident. I'm focusing on myself and working through my codependency but regretting turning down my ex for coffee a year ago. I did it cause I was in a relationship. I'm not trying to come in between but I know he could marry this girl and I do wish I could've changed the timing of things on when we met and all that but I can't. Is there still hope or were we too young that the ship has already sailed and sank?