I feel a bit better today. I'm away from the game for a few days over Easter visiting family, which is excellent for gaining perspective.
The way ahead for me is to change my attitude to raiding and grouping. In future I will be tanking something every time I play - whether a raid, a guild heroic or a PUG. I think this will sharpen my skills so that I'm better at progression raiding when it counts. I will also change my attitude to 25 man raiding I think - I will make a real effort to be there for my guild's 25 man raids (which are generally at dreadful times for a guy with a demanding job and a young family).
Like everything else, I hope that this can be solved with hard work.