I don't want to tell my girlfriend the fact that I watch porn
Is there something bad with watching porn?
I'm just too ashamed to tell her.
I can see being ashamed of the situation. I'm sure she won't mind that you watch porn. Everyone watches porn and I bet she does too.
I will probably never tell her
You are not obliged  to tell her that you watch porn. Wait till she asks you if you do watch porn.