how to deal with heartbreak

Guys have come and gone but I'm sick of feeling so low. They come around and sweep you off you're feet and just make you feel so amazing like the best high, and then they leave not even caring that I'll come crashing down. It's been three weeks since i last spoke to my ex and it's sucks talking to someone getting to know someone every single day to never communicating anymore. He treated me like shit, I know he's not any good for me but I still miss him, especially since he's the one who left me, I don't know what I even did. If anyone was going to end it it should've been me. I know it's for the best but it really doesn't make this any easier. It took me a year to get over the guy before him before I started to even feel better. I really don't want to go through that again. It's a powerless hopeless depressing feeling, please help:(