My girlfriend of 3 months admitted she cheated on me with her ex boyfriend a few weeks ago?

it was a night we got into a fight and she says she did it because i dont show i care. she may be right but i tell her all the time how much i care about and try and do anything i can to make her happy. what hurts the most is that out of all people, it was the ex who was basically her first everything and i've always been very wary of. mind you ive seen a conversation when she had told him she misses him (while we were together!). on top of that, she only told me today (weeks after) because i found something out last night and hooked up with a girl cuz we were pretty much done in my book. now she's apologizing and saying she wants me back and she doesnt know what she was thinking. and that she looked at him and it wasnt the same or anything. she says she loves me so much and it was a mistake. should i take her back? i really care about her...so obviously, i want to believe her. what should i do?? help!