Unfortunately I cheated on my girlfriend again.
Wow, I can't believe it. Why did you cheat on her again?
I just don't feel it when I'm with her anymore.
It sounds like you are unhappy in your relationship with your girlfriend and it led you to cheating. It's not right to cheat on a partner. You might want to think about breaking up and moving on.
But she'll be so upset.
She might be upset if you break up with her, but she'll be more upset if you stay together and continue to cheat. You'll both be happier when you're not together anymore.