I had an affair and now I have no clue what I'm going to do.
Do you know if you're going to tell your girlfriend?
She doesn't need to know because she would leave me anyway
If you knew she would leave you, why did you cheat? Cheating is wrong.
It's natural. Even if it wasn't my girlfriend, I would still probably do it as I don't care about cheating at all.
Whether it's with your current girlfriend or future partners, cheating is wrong and disrespectful. If you aren't happy in a relationship you should be mature and end the relationship rather than betray your partner, who trusts you.