Im falling inlove with a guy who is, i think, really impossible to like me. How could I get over with it?

Go tell him. Doesn't matter if he has a girlfriend. Just go tell him. Most probably he will say no, but it will help you get over him, if that's what you want. I faced the same problem. I was in love with a girl I knew didn't like me, I went and told her and now I am over it and happy.