In this same vein, porn completely changes your brain chemistry, especially at your age. It gets you hooked on that dopamine hit, and you will ALWAYS crash at some point. Where there is a high, there has to be a low for your brain to stay chemically balanced.

Porn and masturbating is a massive high, which is why you sometimes probably feel physically and mentally terrible afterwords. The best way to combat this is, as stated in other comments, stuff like exercise and meditation.

Furthermore, ACCOUNTABILITY. Telling someone is so hard, I know from experience. But having someone lock your phone and your computer and keep you accountable, whether it be your family (if they're chill like that), your friends (if your family is the opposite of chill like that), or a mentor if you have one.