First of all, you need to dump your girlfriend. And I'm not being facetious. If she isn't going to support your dreams now, it will only get worse as time goes on. If she's trying to push you to "grow up", she does not love you.
If your parents have any role in discouraging you, then you need to bite the bullet and move out.
Get yourself a job at a local running store. I did this back in my younger days, and management was totally supportive of my goals and racing schedule. I never hit the big time, but I did manage to improve by leaps and bounds.
You get one life, buddy. Make the most of it and don't let anyone hold you down. If you don't take this matter into your own hands, you WILL -- repeat WILL -- regret it.