I applaud you for recognizing that this is inappropriate and finding a venue to talk about it. This is not a healthy level of rage to hold inside, and you were wrong to treat your girlfriend that way. Do you feel bad? If so, good, you should.

You’re young. I imagine you’ve been through some shit to treat someone this way. You deserve compassion, and there’s nothing shameful about getting help. You should however feel guilty if you choose not to talk to someone about this. At your age you can definitely work through this stuff. It won’t be easy and you’ll have to let yourself feel your pain. But you can do it.