I am more mature than you would ever be, I wish this was a personal conversation. I've had relationships with 23 year old women when I was a senior in high school. I have had more experience in every one of life's situations than anyone my age.
Kiddo, dating someone who is 23 when you are in high school doesn't mean you're more mature. It most likely means she was extremely immature or got her rocks off on banging a high-schooler, and you felt like the big man on campus bedding an "older woman". That 6th grade boy Mary Kay Letourneau (sp?) screwed wasn't somehow so extremely mature. She was a predator, as your "girlfriend" was.
By the time I was your age, I'd spent five years battling a deadly illness, meaning no fun high-school years for me, as well as having to take care of my brother thanks to our parents being extreme alcoholics. They'd been released to my legal custody when I was just 18. Guess what. This doesn't mean I had an adult-level of maturity despite an adult-level of responsibility. It simply means I handled the crap thrown at me. At least I was mature enough at that time to realize that dealing with everything I did didn't make me truly an adult with such a high maturity level as to spit on those who were older and knew better. I'm 28 now, and guess what? I can still see that there's maturing to do. I'm not going to presume myself to be as mature as someone who's 35 or 50. Each stage of life brings with it new things to be learned that further maturity, and these things can only be learned by going through those stages. A sign of IMmaturity is an inability or unwillingness to see yourself as you truly are rather than how you wish to be perceived.
Part of being an adult is thinking through actions in more than a "can I get into legal trouble" way. What happens if this girl gets pregnant? Then what? You talk about being sooooooo mature. If you were six years ahead in the maturity department, how many years ahead is this girl to match your awe-inspiring maturity? She's have to be nine or 10 years ahead, right? For some odd reason, I highly doubt this is the case.
You are both kids. Keep it in your pants and look for someone else who is out of high school.