Why am I still in love with my ex after 2 years?

I met my ex in my first year of university and I fell in love with him hard. He was the first man I ever fell in love with even though I'd had a few boyfriends before him....but he felt like my soul mate. I never thought I would ever find someone like that but I did. I was 18 when I fell for him. I am 20 now and still at the same university. When he dumped me after 6 months of being with him....I fell into a deep depression and started taking drugs subsequently getting drug psychosis and was admitted to a mental hospital for four months. This mean't I missed a lot of my course at university so have to redo the second year again at university. Tonight was the first party of uni and I saw him there. Another man started dancing with me just as I saw him so I danced with him pretending to take no notice of him. When in fact I am still desperately in love with him. I Cannot take seeing him all the time. It breaks my heart all over again. I don't know what do to. I just don't think life is worth living anymore.