I have a secret from my gf
Why would you keep a secret from your gf?
Keeping secrets is a bad habit I have.
It depends on what the secret is, but typically you should tell them everything.
I am not as good at sex as she thinks.
I understand your low self-esteem, but hiding this from your gf is not the way to deal with it. Talk to her and you will see that she will be very understanding because she loves you.