How can I block pictures and scenes of P flashing up in my brain?

You can't.....you don't have direct control over your thoughts and emotions but you have a choice how to react to them when they appear. Try just to observe them and don't jugde yourself for getting these thoughts. If you observe the thoughts as the observer your point of view will change. Keep doing this and your thoughts might dissapear.

 

Its normal to have these type of scenarios when you are rebooting. See thorugh years of exposure of PORN and MASTURBATION simultaneously to your brain its been wired to a certain state, it will take time to get rewired.

Apart from this I don't know any mind tricks for now, but you an diversify it by trying like countering with positive thoughts, thoughts that will encourage you. You can try learning MEDITATION, this helps a lot. In many ways it will help you not just by combating this situation that you are facing right now. Another important thing I saw in a TEDx video that apart from asiding crappy feelings to overcome it we have to walk through those. If I am able to get that link I will post it here.

As for now calm yourself down, thoughts will appear you just try to ignore them, think of what lies ahead of you if you combat this situation. We are all with you, just get going.