How easy is it to become the boyfriend of a girl currently with a boyfriend?

There's this girl who has been with the same guy for about 2 years. She is 18, I am 19. I have made a really good impression on her, I can make her laugh, smile, entertain her, show her i'm confident and all other good characteristics. I really like her personality and like hanging out with her. So far I think I am impressing her and would make a better boyfriend than her current one as I have met him and he was really quiet but he must have some good qualities for her to be with him so long. How hard will it be? I think it will be really hard seens as she is already in a long term relationship with him. Any tips?

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