How do you get over a girl?

I know i tried on 4 different occasions trying to get to know this girl because i really like her. all 4 tries failed. the last one came when she didnt show for a lunch on campus. after that point i decided to move on and keep it casual. i have. funny thing after that she started asking me to study. i think she is trying to be my friend. i know on two occasions we were suppose to meetup. the first time was awkard in a sense that i dont think she really wanted to me even though she ask me. the second time we didnt meet although again she asked me. she end up spending time with anothe guy which wud ultimately end up being her boyfriend. i trying so hard to keep it casual and friendly. she is still nice to me and all but i think she doing after the fact. how do i just keep it casualHow do you get over a girl?
There are so many girls out there in the campus...make use of the opportunity. She already has a bf and is telling u obviously that she juz wanted to be a casual friend to you. So keep it casual and give yourself a chance to get to know others...move on!How do you get over a girl?
Time heals all wounds :)