Ok, so me and my "best friend" have started dating, Ive known her since we were like 15, and were both 21 now.
Ever since school, we've always treated each other in an agressive, playful way. Like insulting each other, but playing, never seriously. Now, 7 years later that we have started dating, I keep doing it, I keep treating her like I used to do 7 years ago. And I realize that she responds the same way, and I guess I feel bad, because we're "together" now.
You could say Im not calibrating to what I want to have with her, we've been seing each other for about 5 weeks now.
How can I go from treating her like a middle school crush, to treating her the way that I want to have now an adult relationship??

Nothing wrong with playful banter, as long as it's not hurting either of you. Keep things playful, why not, if that's the you she's attracted to why change?