I find having the mindset that this is an opportunity to be a mature, safe cyclist and to demonstrate good cycling practice, rather bloody hell I'm being held up helps reduce the stress in these situations, and to actually accomplish all the obvious advice re. Highway code etc.
I have also found passing other cyclists needs extra care, as motorists don't expect cyclists to pass oneanother (I think magnatom has a video of a man in a red civic agruing about not cycling two abreast when he'd actually been overtaking another cyclist). Good practice for me is clear and long rear checks, good signal, and then super hard primary (next to the centre line in a narrow single carriageway), being sure to remember rule 163 applies to bikes passing bikes as much as cars.
If you still find it hard to get out of the 'being held up' mindset, perhaps regard it as an enforced rest period before a sprint interval where you regain any lost time?