I’m sorry for your very genuine pain. I’m sure it hurts. I believe your regrets. But it looks to me like this has been a long time coming.
I don’t know what you mean when you say you don’t know how to express yourself sensitively. If it means that your girlfriend hasn’t felt loved and respected for some time, telling her you didn’t love her was the “last straw”. it gave her the push she needed to end the relationship with you.
For your girlfriend to find someone else so quickly suggests to me that she left you long ago. Even if you can be convincing in showing her your love now, it probably won’t be returned at this point.
Do work with her to figure out how to continue being a father to your son. He needs your love to grow to be a loving man. You and his mother can still be loving parents for him even if you can’t love each other. The boy deserves it. Fathering well will help you develop the sensitivity you say you lack.
I wish you well.