I'm going to see a therapist behind my gf's back.
Why would you need to do it behind your girlfriend's back? Why does she care if you see a therapist?
Because it's about something personal I don't want to discuss with her.
That's completely fine if you would prefer to talk about the issue with a therapist, but there's no need to hide the fact that you're going to see one. I'm sure your girlfriend would be fine with you seeing one, and would understand if you have something personal that you don't feel comfortable discussing with her yet.
Yeah the problem is unrelated to her so hopefully she will be ok with it.
I think she will understand that if you tell her that the problem is not about her and very personal. Be up front with her about seeing a therapist though so she finds out from you instead of learning you tried to keep it from her.