How to get rid of other guy?

So, basically there is a girl i love, she is my classmate in the university and we have a lot in common so i thought everything was fine, but

there is 1 guy who i know for a while and he started hitting on her and they are very close now, chatting hagging and doing these staff

i am in a bad condition when i hang out with those 2 i am seen as a friend of him and i don't know what to do

how can i change our relationship with the one i want?

How to get rid of other guy?
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