You cope by transitioning, to the extent you need to in order to cope. You have gender dysphoria, in the literal sense, if not in some sense defined in the DSM: You are feeling bad on account of the incongruence between the gender the world sees in you and the gender you want to express.
If you're interested in transitioning, you should start as soon as you can, in whatever way you can, because the best way to figure out if it's really what you want to do is to give it a try. Any steps toward that goal — therapy is good, especially with a therapist who has a clue about gender nonconformity. Also, local support groups can be good — actual in-person (not YouTube, not forum) trans people. But any gender-affirming things you can do, that feel safe to do (even if they're a little scary), are steps that can help with coping.
Try to tell your girlfriend. This is affecting your life really seriously, and she really deserves to know, and she can help if she wants to stick with you (whether as your girlfriend or as your girl friend), and if she doesn't want to stick with you, she can help by leaving. That is, assuming she's trustworthy. If not, well ... I guess it's not such a great relationship to be in?
Obviously, if you are in a dangerous situation where you risk becoming homeless if your parents find out, address that first: Think of independence as a step toward transitioning.