I haven't been riding audax long (about 12 months) and its been a steep learning curve, particularly the mental side. I am only partially of the way up the curve (or rather, I think I still have lots to learn!) but some of the issues I have faced are:
1) Pacing, lack of discipline = mental depletion
2) Unrealistic expectations that then unwind mind-ride
3) Thinking that the overall objective is "enough". By which I mean that although I really want to complete a 600km, once I am tired at 150km and I have 450km painful/cold/wet kilos to go, the imagined satisfaction of a finish isn't enough and I sink into "why the xxxx am I doing this?"
As per already stated, my answers aren't perfect, and mostly they are based on friendly advice I have been given:
a) I have got better at pacing by riding my own ride, and letting go of faster groups. Strava helps.
b) I am trying to learn a bit from my old job, and have upside and downside plans.
c) The advice from guys who are well ahead of me is to try to ride in the moment, enjoy (or at least be at peace with) what you are doing. There are different ways of saying this, I think (and I'm just learning) but it boils down to this: How good/bad do you really feel? Can you ride another 10 minutes? Keep turning the pedals, and you eventually crest the (mental) hill.
We have all chosen to ride our bikes - there is little sense in thinking "I'd rather be sitting in front of the telly" because when we were rational(~ish), we chose to enter the event instead of sitting in front of the telly.
Finally, there are times when we can all predict when we will feel miserable. For me, its rain and the long-term affect of pedalling too hard. For others, maybe its a certain number of kilos done, or kilos to go. Have a plan for this too - take a little pack of Haribo or a Crunchie for just such an occasion, and smile inwardly as your rational, thinking self kicks your inner chimp in the danglies!