Unfortunately there isn't really much you can do. If he doesn't feel for you in the romantic manner, there isn't anything you can do to convince him. I'm sure he liked the attention, as we all like the ego boost of somebody liking us.
How did you find out he had a crush on you when he was seeing somebody else? Did he flat out tell you that while he was dating that somebody? That's a sketchball move if he did.
I know it's weird, I know it hurts to have these feelings when they don't feel the same way and there isn't a magical way to fix yourself up. You have to find things to do on your own, ways to make you happy, and soldier on.
They are the only person that can make you feel better, but there is no way to convince them of that and have them do so.
Waking up next to a beautiful girl,
Step outside and say hello to my beautiful world.