The first thing to say is that your feelings are entirely normal. It is a huge shock to be diagnosed with cancer, and waiting for test results is, in many ways, the hardest part of the process. You have a staging and a Gleason score so presumably you've already had a biopsy? Do you know what the MRI is for? Just of the prostate or a whole body scan?
The "good" news is that T2 G3+4 cancer isn't too bad. In the vast majority of cases it's completely curable, and it's very, very unlikely to kill you. The treatment options aren't pleasant, and they can be life-changing in terms of their effects, but you will get through it and start enjoying life again, difficult though that probably is to imagine at the moment.
I'd suggest downloading the "Toolkit" from this site which describes the various treatment options, and also talking to the PC-UK nurses who are wonderful people who can answer all your medical questions.
So try not to panic. You've not been given a death sentence; you've just been diagnosed with a disease that needs to be treated. We've all been there and know exactly what you're going through. You're not alone.
All the best,