Can anyone tell me why I always have a relapse

Simple solution.
1. Keep yourself busy doing something you love or learn something new.
2. Always motivate yourself, whenever you have an urge just think about the future. Is it really worth it to fuck up your next 30-40 years of future life for 5 seconds of PMO. ?
3. Start taking cold showers.
BEST OF LUCK

 

Spoiler: About Relapse Prevention

 

See if you can find what's the cause of the urges. For me it's mainly when I am stressed/sad or when I have an argument with someone. Whatever they are try to avoid them as much as you can.
Also as the others mentioned, you need to remember. If you forget it try to write them. If you clearly remember your urges in the past and how bad you felt after relapsing / how good you felt after not doing so, then it may help to encourage you to not relapse. Save these incidents and remind yourself.

 

As much as many people might like to think so, your body doesn't just act out on its own. It takes a decision of the mind. The mind is the king over the body, and the body merely follows the orders given to it. If you are relapsing, it is by your own choice. Can any one of us tell you why you are choosing the way you are? An educated guess would be simply that, having failed to put the thoughts out of your mind, and having just resigned yourself to a life of such failures, you simply cave in to your addiction without putting up a fight. To stop going down the road of addiction requires willpower. It requires decision--and planning ahead. If you truly decide to stop smoking, for example, you make sure that there are no cigarettes anywhere nearby at hand. If you want to quit masturbating, you must similarly not even let your mind consider it as an option. Keep the thought well out of your mind. Don't go anywhere near the places or circumstances where it would be more difficult to resist. Stay as far away from the edge of the cliff as possible.

But, as a Christian, I also think it is much harder to stop without God's help. Some non-Christians have sufficient willpower to stop their undesirable actions--often to make an impression on others. But without God's help, the heart's desires will not change. It's so much easier to change one's desires with divine help, then, when you truly do not wish to do what you used to do, it's a cinch to make the change. You change from the inside out, rather than the other way around--a bit like the ol' cart before the donkey dilemma.

There are some links in my signature with more detailed tips for quitting the addiction.

Hope this helps!