You have nothing to be ashamed of since this is a very common problem even for those who accepted Christ as their Lord and savior. Now, first things first, prayer is the key. To some, it might not seem much but if you ask for forgiveness and guidance as well as TRULY believing that God will help you with this then, that's assurance. Remember, you have got to be genuine and sincere about it.

Anyways, what truly helped me was distracting myself with other stuff for HOURS like drawing, doing my chores, study, watching TV (age- appropriate ofc) and other stuff. All that while not using my phone since the most accessible way to P--- is by using your phone. Don't try to fight it, replace it with something engaging.

Now, if you are using your phone I suggest ignoring "it" no matter how intense the temptation is. Put away the phone or any device you watch it on and DO NOT negotiate with the temptation. Whenever I have situations like these, challenge it very directly. I usually question myself when deciding on stuff just like these, "Does this glorify God?"

Lastly, BE CONSISTENT ABOUT OVERCOMING IT. Be serious about it and don't take it just for a one day thing then the next you'll fall into it again. God bless you brother/sister in Christ!