The reason you relapse is because you still hold subconscious beliefs that porn is awesome and belongs to your life. You need to change those beliefs or you will keep relapsing. That is done through active self-reflection of what porn really is and what its true benefits and cost are.
When we quit something that was such a deep part of our life it takes time and lots of brutal honesty to make your whole being understand that it's bad. Otherwise it's just like a third of you that wants to quit but the rest still wants it, so you have to fight yourself every day, and of course every now and then you'll lose which means you'll relapse.