I'm not going to tell my girlfriend I am transgender
Why do you want to keep being transgender a secret?
Because I don't want her to know.
It's important to be honest about yourself with your girlfriend. She'll eventually find out that you're hiding the fact that you are transgender from her, and she will feel hurt that you didn't trust her. If you being transgender is going to be an issue for her you'll want to know before you get to far into the relationship anyway.
I don't care. I don't want to tell her.
If you care about her, you must care about her feelings. Honesty is the best policy when it comes to relationships.