It's easy to "function" on the surface. People do only want to 'see' that, and feel that you are coping, because otherwise, they don't really know what to do. They certainly find it hard to help, despite the best intentions, because our experiences scare them. Bad things do happen.
I still cry most days - in shops, at home, with friends, alone... it is just part of me, and a way to honour my love. That is exactly what you are doing for James. Don't take it as a measure of weakness, or not coping. My husband calls tears "liquid love", and he is exactly right.
Being irrational is perfectly understandable. In any normal world, your circumstances, living beyond your son, is irrational. However, if I can suggest, it isn't good to bottle it all up. Perhaps it is time to revisit your counsellor and find some ways to work through these feelings?
But please, please - of course you aren't failing James. You have his diaries, you know how self-aware he was. He was in the grip of a terrible illness, one which distorted his view on life. Neither you, your family, or even James himself is to blame. He was just desperately unlucky.