What you did is called a lie of omission. You did fuck up. Not by remaining friends but by not telling someone you entered into a relationship with that you had sex with someone you currently lived with, and two people you spend time with on a regular basis. Don't do that again. You should remain friends with your ex-fuck buddies or ex-lovers or whatever term you use (not trying to be offensive). You'll need them. But next time you want to enter a long-term/serious relationship with someone you need to be honest. You need to let them know you used to fuck your friends, for a long time, so the new person can decide if they want to be in a relationship with so many strings attached.

How would you feel if the shoe were on the other foot? I don't think your girlfriend will take you back and I hope she doesn't. You hid something huge from her because you didn't think she would accept it. She will never forget this and will always wonder what else you're hiding or if you'll hide other important things in the future. That's a feeling you should pay attention to in the future. That was the clue that you were doing something you knew wasn't right. I'm not going to give you points for finally admitting to something she should have known about before the two of you got serious.

Try to work on feeling better, know what to do in the future. We all fuck up. Learning from it and being better in the future is the best thing you can do.