How do you get over someone? *serious question*?

When you really love someone, how do you let them go? I'm being serious.. My boyfriend and I were going out for almost a year and a half. I loved him dearly. I understand that people have gone out for much longer than that with their significant other and they were able to get over them.. but I don't know how I will be able to. It has been almost 5 months and I just can't let go. I really did love him. I broke up with him (he was starting to treat me badly so I thought it might wake him up. I understand that it is wrong to do things out of spite, but that's what I thought the best choice was so I went for it.) He thinks I cheated on him, and believe me I have clearly told him that I didn't and I have expressed to him that I meant everything I said to him, I've tried everything to show him that I actually did love him and I would never do that to him. I have even sent him a letter. But he refuses to talk to me and he has been acting completely out of character for the past couple months; drinking ridiculous amounts of alcohol, getting high every weekend, he has done ecstasy a handful of times, his grades are dropping and he used to have a 4.0, and he is getting suspended from school along with 6 other guys. He used to be everyone's best friend, the funny guy, the athlete, my world.. and now he is going downhill so rapidly. It is making it easier for me to detach myself but it's making it even harder to stay away because I do love him dearly and I want to help him, but people have said this is his way of dealing with the pain of our breakup.. Plus, he has a thing with this one girl who is fairly pretty but she is a druggie and not too many people are fond of her. Plus, he has hooked up with multiple girls so I am not convinced that he likes this girl too much. Especially since he isn't finished resolving this relationship considering he can't even look at me let alone speak to me. I have no idea what to do because I want to move on, I'm just not comfortable with it because I am still attached to my ex boyfriend. Please help? Any advice would work, I would really appreciate it thank you. (: