My advice, start hrt. Don't worry about tellin anyone if you don't want to. It's no ones business but yours. If you need to, consider getting some therapy. If you need to, consider going to some type of support group or something. It's your life though. It's your body. No body else matters. As for your girlfriend, again, when you're first starting hrt, it's none of her business. You should tell her only when you feel like it's relavent to your relationship together. You can be on hrt for a while before anyone will notice anything. Anyone who sees you everyday won't pick up on the changes much. Just worry about you. In 20 years, you're the one that has to live with the consequences. Do you think she'll be around in 20 years? Every trans* person I know says the same thing a few years after transitioning. "Why didn't I start earlier?"
I'm no different. I wish I'd pushed harder to start hrt earlier. You don't need to transition socially to take hrt. But, it makes a world of difference. The younger you start, the better it will act on your body.