Got cheated

M/23 my gf cheated me last month, she was all good before 2month but suddenly I saw some changes in her behaviour than slowly I got to know that she got new male bff when I asked about him she said just a casual friend and in some days she dumped me without even saying any reason. Now she wants to come back to me and I don't want her back cuz when I used ask her for sex she always said no but with that guy she did it, that's the reason I don't want her back,what should I do accept and forgive her or reject her and go far away from her.