Should I take back a cheater?

Okay guys, so I had this girlfriend and I recently found out she cheated on me. She was 17 and she cheated on me with a 21 year old at a party and I didn't find out 3 months later untill her bestfriends boyfriend told me about it. I questioned her about that party and she ensured me everything was alright. I've been talking to this new girl, she seems pretty cool she's 20 and I am 18 but I still love my ex. She said she would never drink alcohol without me again but it's just hard to believe because she lied so much. I love her, but I don't know if shes worth it. Help?