My cheating ex-girlfriend wants me back, can I get some assistance?

Okay, I'll make this as understandable as possible, I'm 18 years old and she is 18 years old. I dated my ex for a good two years, we weren't the most perfect couple, we would fight often and breakup all the time, but at the same time we were in complete love and were eventually planning on marriage in the future. It was around the end of last year when I got a feeling that she was cheating (lack of physical affection, never wanted to hangout) and using my knowledge in computers and used a keylogger on my computer when she came over one day and retrieved her Facebook information to do some browsing. To no surprise, I found pictures and messages to some older guy around 22 or so years old. I was heartbroken... I felt sick to my stomach and didn't know what to do. I brought it to her attention and we parted ways. At first, I couldn't take the pain and I would always ask for another chance and practically beg her to take me back. She wouldn't budge and before I knew it, she started dating this guy. I was jealous. They could only see each other on weekends due to the distance, but even after that... we would hangout everyday. We would makeout, have sex, and pretty much still have some physicalness. She was cheating on this guy with me basically. So he found out we still talked and told her to stop, we stopped for a little and she cracked and told me how much she misses me, he found out and got my number and texting me telling me to back off, I told him everything we do while he's up in college and he was fuming and admitted to me that he just used her for sex. He broke things off with her and now after months of on-and-off talking, I've tried to keep away from contacting her, but from looking at her Tumblr/Twitter/Facebook, it's clearly evident she loves and misses me. We go to the same school and lately I've been giving her more chances to make things right, yet she refuses. Her tumblr/twitter continues to prove to me how much she misses me still as they have breakup/love song lyrics, memories of our past, and pretty much everything your average female would miss in an ex-boyfriend.

This is where you guys come in, I don't know what to do. I love her and miss her so much, but am I setting myself up for more hurt? My friends want me to move on once and for all, but I just can't and they aren't to happy with me. Is it worth it anymore? Do you guys believe in "Once a cheater, always a cheater?", and do we try to make it work?

I want her back, she wants me back. I treated her like gold and I just want things to work, but don't know if I should even put in anymore effort.
Updates:
+1 y
I meant to put this under break-ups.
My cheating ex-girlfriend wants me back, can I get some assistance?
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