As a coach managing what an athlete "thinks they can do" versus "what you think" they can do can be an issue. Another is performance... you get some athletes who do really well and think they haven't.
Let's take a look at the initial consideration. It's obviously not a great idea to tell an athlete when they say/think they'll jump 8.50m next comp when their pb is 7m. (I've made this up BTW; no one I coach has said or thought that... mind you if one jumps 8.30m, I may come back to this post!). I generally try to remain positive and tell the athlete to focus on the process rather than the outcome and this applies to a number of athlete/coach performance issues/discussions. If an athlete tends to focus on the outcome then there - in my experience - is a likelihood of a negative result. This is why, I guess, selectors for teams, don't seem to like athlete chasing qualifying marks (however, it does beggar the question "well, how are they going to get the time/distance?"). Trying to focus on process i.e. technique will hopefully induce a better flow state than a focus on "going for it" (going for it is fine when you are in a flow state!). Tension breeds stress and this type of stress can impair performance. So I'll focus on key cues that we have been working on in training for example, for the triple jump running through the take-off and going long and waiting for the hop/step contact... hopefully this will manage unrealistic expectation and bring the focus back to realistic performance - performance which can be achieved.
And what of "I did crap" - when the athlete didn't (I'll address what to do in another post, if they did). Now sometimes healthy expectation is good in this respect. The athlete however, with guidance needs to contextualise what's happened - to really see the positives. I've had athletes reach finals of international competitions and been disappointed. Now, this is all about that word context again. If the athlete is a young athlete and has managed to perform with credit (whether that be on the podium or not) then this needs to be focussed on, and the view that the experience is a stepping stone to the next big event emphasised. If the athlete has - in your mind performed very well - but they believe they haven't, then it's a case of perspective, of educating that athlete as to why they did well (and touching on in time, where they can do better. - this does apply to all related scenarios) I also know from experience that the athlete-types who often feel they have underachieved when in reality they haven't are the ones that go on to then win or medal or Pb when they reach the similar level of competition again. It provides the drive. And this indeed is the conundrum... you need "fire in the belly", but this needs to be tempered.
It's often the athletes who responsibly blame themselves i.e. accept responsibility for a bad performance who are the ones that will be more successful. Blaming external factors seems to absolve some responsibility. To perform well you need to control what you can and deal with what you can't.