Her family and friends are all telling her she shouldn't get back with me. What kind of people influence someone to cheat on their boyfriend. I have no one on my side, my mom was like her mother, and she's ignoring her too. I don't get it.

LISTEN, this is the last answer I can actually right, and I hope it will make you feel better.
There are two ways you can see this. The good way and the bad way.
The bad way is the way you are seeing it right now.
The good way would be admitting that you had great times and were great together. She will never forget you because you were her first true love. So don't take away the worth of what you had together.
Wake up in the morning and get the hell out of bed, and live every minute of your day like if it is the last. Life is too short to dwell on things that are not worth it.
Think of it as being a good seven years. But you still have at least 60. Guess it doesn't seem that much looking at it that way. You learned a lot from your mistakes and this will only make you a better person in so many ways. So stop looking at things negatively and start appreciating the things it brought you. LIFE IS WHAT YOU MAKE IT! Remember that.
Give value and respect for the time you were together, it is so much better and will hurt a lot less over time than hating and wanting revenge.
And the feelings you are feeling are nothing but normal to any human being in your situation. You're not less of a person because of it. You have true feelings and in time you will heel.
So please take care and keep your head high. Appreciate what life brings you because life has unusual and hurtfull ways of showing life lessons. But in the end, it's all worth it.
Hope I helped!