What do you do when you still have a crush on your ex boyfriend ?

An ex is an 'Ex' for a reason sweetheart.

X Dani

Why is he an ex?

Well, I'm guessing there is a reason why he's your ex. I would think about that reason and if it is unable to be resolved I would continue to have him be an ex. Unless he has feelings for you as well and you both want to pursue the relationship there is really nothing else to be done.