I'm still in love with someone who doesn't exist?

Hi, I'm 18 and 2 years ago, I went out with a guy 2 years older than me and I really liked him. He went off to college and so we split up and all I could do was think about him. When he came to visit, he changed. He's depressing and completely different. It's been so long and I've tried being with other guys but all I can think about is who this guy used to be and how much I wish I could go back. Any advice? Time isnt helping