I understand what you are saying, superkid555. I'm also 15 this year.
According to the Bible, we as Christians go through many 'storms' as Christians. In this case, it is a struggle for purity. I have face it too. Many times, even now.
Christians aren't people without problem, but people who go through them, people who get them solve with God's help. Storms in life are meant to strengthen us, not put us down. God doesn't want you to run away from the problem, but to go through it. He will be with you, "
for the LORD your God goes with you; he will never leave you nor forsake you." - Deuteronomy 31:6.
Temptation will never go away, it is always tormenting us. But God has grant us the power to overcome it. Even Jesus was tempted by Satan. But Jesus use the Bible against Satan, as shown in Matthew 4:1-11.
Now on to ways to stop doing ' y'know ' :
1. Tell someone close about it
-It can be your parent(s), close friend, pastor, etc.
-It is very hard. I know. But, abandon your pride, just 'kill' it.
-Remember to tell them that you wanna stop & ask them to help you, so they don't get the wrong picture.
2. Anytime you get the 'urge', by all means, stop it.
-You can try shout in a pillow over your face or something.
-Having a hobby and doing 10 push ups everytime you get the urge, as advised by cybertech is also a good idea.
-Don't inflict pain on yourself, it is not advisable.
3. Understand the consequences of it.
-This link may help : Why I stopped watching porn: Ran Gavrieli at TEDxJaffa 2013 - YouTube
4. Pray
-This is one of the most important step, asking God for help. Credits to MooseAura & cybertech
It is very good that you have already done the first and most important step, that is, to recognize that doing it is wrong. It is also good that you have come to this website to get help. There are many ways to stop it propose by others on other similar threads.
Paul has also wrote in Romans 7:15-25 about the struggle for righteousness :-
"For I do not understand my own actions. For I do not do what I want, but I do the very thing I hate. Now if I do what I do not want, I agree with the law, that it is good. So now it is no longer I who do it, but sin that dwells within me. For I know that nothing good dwells in me, that is, in my flesh. For I have the desire to do what is right, but not the ability to carry it out. For I do not do the good I want, but the evil I do not want is what I keep on doing."
May God guide you & bless you.
Good luck, superkid555.