I don't know how to answer 'how to deal with anger?' I do understand anger, if someone is mean to somebody else it makes me angry, but that is because person A has a choice about being mean to person B, and person A makes a bad choice. I view cancer as a random event so there is no one to be angry with. If one believes in a god, one might be angry with her, but I don't believe in a god. You can't be angry at nature, because nature doesn't have free will.
I can understand a bit of anger towards the GP, but GPs do not understand prostate cancer has no symptoms til it is too late. That is a training issue, a GP can only use knowledge from their training. You perhaps can be more angry at those who train GPs and this would open up a question about screening (can I beg of our fellow posters to not use this thread for a screening debate, search for old screening threads, read them, and then either post there or start a new thread, this thread should be reserved for 1982 CCC dealing with her anger).
You are right to recognize your anger at people whining about trivia, when matters of life and death are of more concern to you, is a you problem not a them problem.
I've just seen decho's post, yes support groups like Maggie's or indeed online like this can help. Medication (sertraline is common) can relieve anxiety and depression, and these emotions can be contributing to your anger.
I don't know if any of this helps. Just because my outlook on life works for me in this situation, doesn't mean it works for me in other situations, and I don't expect you to change your outlook (is that even possible). All I can hope is that you can see there are different ways of looking at life, and some cause less negative emotions.