Why am I still so bad at chess?

Hey All, 

I've been playing chess on and off for around 2 years (still a teen) and a couple months ago decided to make an account on chess.com. Since then I have played around 150 games, and my question is this. Why, after watching many different chess resources, analysing my games move by move, doing puzzles, chess.com game reviews and more am I still so bad? I believe I am relatively intelligent (not a genius or anything, but done well in school without any real effort), but am struggling to improve my chess.com score (while I understand this is not a particularly accurate measure of chess skill, I have not been improving, and in fact been losing rating, and I was wondering for anyone who has experienced anything similar or has advice regarding this, how can I improve? I have the lofty goal of achieving a chess.com rating of 1800, and currently am nowhere near that (rating approx 750). Any tips or advice would be greatly appreciated.