I keep living in past.

I really need some good advice on this issue. My ex broke things off with me 3 months ago, we were together for 3 and a half years. Since then i've been feeling so bad. It comes and goes in waves, sometimes I don't think about it as much but he is somewhere in my head all the time. I could be doing anything, and i'd still have a little bit of him on my mind. I keep living in past and idk how to go on. This really became an issue. I know he's on okay terms with his ex right now, they were together for maybe 2 weeks and i feel so so so bad. We talked a few days ago and he's so mean to me. One moment he's fine, then the other he's so mean and he blocks me on all social media. I feel like i'm on some drugs and i can't have any. How can i stop this? I just wish i could forget about him completely. I also keep checking his social media. Can someone please help me?