It seems as though it should be so simple..
It IS simple, just not easy. I too relapsed too many times after vowing to stay sober. I tried to learn from my mistakes, forgive myself, and move forward again. Have you given any thought to a recovery program like AA or one of the other programs? What about counseling? Use any recovery tool available to you. Keep trying until you can stay sober. It helps too to keep in 'just for today'. 'Forever' is a long time, but 'today' is manageable.
Welcome to SR! I'm glad you found us!