well, why do you feel guilty? is it because it conflicts with your beliefs? or is it because you have been told it is something bad?
If it is because of your beliefs, it is better to follow your faith than to live with a guilty feeling (it is possible to stop... but only if you can look at the big picture, and only if stopping is something you really want)
If it is because you have heard it is wrong to do it, I know you have probably heard it from people who oppose it because of their beliefs... if that is the case... what other people think about something doesn't have to be what you believe about it...
I try to look at things this way, perhaps it will help you too...
If I feel guilty about something, I try to find the root of why I feel guilty...
when you find the root, you usually can know what to do from there...
Listen to your heart... at the end of the day it is your choice, your body, your feelings, but life is all about the choices you make... everything comes down to a choice...
about the pornography, usually young girls will watch it because they are looking for something they long for, young girls long to be in love and to be loved by someone, so sex is often misunderstood for love...
I used to be addicted to pornography myself when I was your age,
But it is empty... it doesn't make you feel good at the end of the day (well physically maybe) but not the heart...
I am not telling you what to do, the choice is yours,
I am also not telling you that masturbation is right or wrong,
there is no answer to that (only that sex is meant for two people to enjoy together, and masturbation feels a little lonely doesn't it?)
the only thing I must say is that watching porn is addicting and addictions are not good emotionally and physically... talk to someone you can trust maybe to help you, get a really good hobby, an activity, set a goal perhaps, anything to take your mind off of it is really helpful if you want to stop, leave a note on the tv or computer (could be your own secret language) telling you not to watch it...
I hope it all works out for you :)
and remember live, learn, laugh, love... you are young and have your whole life ahead of you, you are strong, you can do anything you put your mind to! :)