Lots of useful hints there muppitz. If you've been playing the guitar for a number of years and can accompany songs then you ought to be competent by now, so maybe it's a lack of confidence. I'd be very tempted to find out the names of the most popular tunes played at sessions 'Atholl Highlanders' is a good start!! and choose two or three to practice at home to a CD etc until you are really confident with the chord progressions. Then it's a matter of transposing the key to whichever is used at the session & listening to the rhythm. Maybe starting with a slower tune might be useful. Once you're confident with a few tunes & not making any major bollocks then your confidence will grow. Also remember that the other guitarists in the sessions might not be playing the right chords/key/rhythm either!!! (been to a few). Your ears are your most important asset, use them & good luck.