How do i tell my girlfriend i don't want to be sexually active?

Its great you have that mentality of not wanting sex for the betterment of your long term health. it's important people recognize that it's not sex that is bad, it's the abuse of sex and using it for over indulgent, selfish reasons that's the problem. until you can realize that in a relationship sex should be intimate and special, you will have a hard time with a healthy one. your recent knowledge and willpower to stop this indulgence is a great sign that you realize it's the unhealthy part of your mind and you are trying to heal. one day in the future you will be able to have both a mental and physical healthy satisfying relationship/sex due to making love and not just orgasm. stay well!