I'm happy you could get things off your chest, hope it helps, even if in the smallest of ways.
Truthfully, it sounds like due to circumstances and timing, you guys broke up and you regret it now. But she's with someone else and probably happy. I know what it's like to be hung up on an ex, I did the same thing when I was a lot younger- kept wanting to get back together with her coz she was so perfect, and how could anyone else even come close? Just couldn't envision a future with someone else.
But she moved on, and it was probably for the better. I wasted time that I should've better spent taking care of myself, focusing on my own personal and professional life. I'd suggest you try doing the same, no matter how impossible it seems right now. I know it might seem incomprehensible right now, but you can find happiness without having that person in your life. Sometimes things aren't meant to be and two people who might seem very compatible don't work out in the long term. That's just how life is, but you gotta get through it and things turn around again, I promise you.
Don't make your whole life center around one person or relationship, it's really not healthy for your own self-worth. Invest in yourself and you'll eventually see things differently. I don't wanna be patronizing here, but you're still young, and the way you'll look at this in say 5 years will be different from how you're seeing things now. Letting someone go isn't easy, but can be necessary, and you'll find happiness again once you're able to accept it and truly move on. I think you never had a sense of closure with her, which is why you're feeling this way even more.
Not much else to say, good luck with your future. Please take care of yourself and try not to self-harm or contemplate other things. There's a life worth living for yourself and I hope you see that.