girlfriend of 2 years, recently been acting a lot differently than usual you aren't the most attractive person but your far from ugly

her: 20, your gf, bi, youve been dating for 3 years, loves and cares deeply agout you but has been cheating for the past month you:,21, (make your own hight and sexuality), wants to marry her, oblivious to her cheating, has been cheated on a few ties with other women story: you went out to the mall looking rings for her becuse you are going to propose soon when you see her walk around with another guy, holding hand and staying as close to him as possible (what do you do?)