The best way to boost confidence is to set goals and achieve them, which is what you seem to be doing already.
Confidence can be NOT saying anything to the jerks that are hitting on your girlfriend. It shows you are confident your girl can handle herself, you see?
You can also show you are confident by NOT running your mouth back to guys who are immature enough to try it with you, it shows you are more mature.
When you his 23-24 your brain goes through a second re-molding, it takes on information better and faster, and you will mature whether you like it or not. It's like you will learn all the lessons life has been trying to teach you for the last ten-fifteen years, all within a few months. (i'm not sure what this re-molding is called, but I did read about it in a science magaine, and i'm explaining it in layman's terms, so bear with me)
I was beaten severely by a group of 20+ year olds when I was 16 (I was a tall kid) and it stuffed my confidence. The way I got it back was to earn it - I put myself in situations in which I had to front up some courage over and over. And now I'm not scared of shit, I'll happily do public speaking and I'll smile as I walk past a group of patched gang members down a dark alley. You need to GAIN respect for yourself, and the only way to do that, is to do some things you consider 'hard'.
Sorry about the huge reply, welcome to the PG