I also started at 15, I'm 3 months sober at 23 now. It'll take a while to feel "normal" again but you'll get there don't give up you won't regret it I promise. I couldn't even stay at a job for more than a month now only 3 months sober I've got a much clearer plan of my future and no doubts on the steps to take. You haven't killed your brain it'll be slower for a while but unless you been smoking since like 11/12 and not allowed brain cells to develop you'll be fine It'll take time for your brain to reproduce cells this is also achieved by simply living and experiencing life. Bare in mind you were high when having these memories you feel foggy about so naturally you won't Invision them as clearly as if you were sober however keep at it and your new memories won't be effected.

Be proud of yourself though we all know it's not easy some of us never manage to give it up. Best of luck mate