Most Helpful Girl
You've already started an affair by doing it more than once. Obviously you didn't regret it the 1st time. The way to stop it is to stop being selfish be honest with your girlfriend and stop talking to the other female block her off. And if you can't find yourself doing it. Than leave your girl and let her find someone better. If you have to choose between 2 people choose the second because you wouldn't have had to choose if the first one was the one.
You cheated the first time you had a one night woth her. You then created an affair when you did it again multiple times and carried on with keeping contact with her.
Most Helpful Guy
They're the same thing. You're stupid to do it and you don't deserve a relationship. Beg for forgiveness and don't expect to get it.
To stop it, realise that you are in total control of your body, where you direct your feet, where you place your hands, the words you say, YOU are in charge of every action your body takes. You don't have to act on every thought, you choose which thoughts to act on. Now, stop.
You probably feel it's hard to control your thoughts. You're right, that's harder. But each time you take a step in the right direction, you make it easier for your future self to keep going that way. (Equally, if you make the wrong choice this time, you're making it easier to go wrong next time.) So even if you can't completely change your thinking at once; and even if you sometimes go wrong: Keep thinking the right thoughts as best you can, and every single time you do that, you're making it easier.
But yeah, regarding your actions - just stop it.
A final tip - Admit everything to someone you know, someone who will be disappointed at the news but will not reject you from their life. (Maybe a family member?)