Just getting your career to function in simple economic terms is difficult in itself, but the incessant travel (more of a strain nowadays than ever), the socializing you have to do after so many concerts, the simple logistics of getting to the hall on time, not forgetting any of your music or gear, dealing with halls that are too cold, too hot or just too dead acoustically--and on and on!
And then you are finally there, walking onstage, in front of the audience. You sit down, ready your instrument. And...
You forget the music!!
There are many accounts of this horrible event, the bane of any classical soloist. I have some myself. For example, on one occasion I decided to try out a fairly new piece in a low-pressure lunchtime concert. The piece was this one, Madroños by Torroba:
What happened was I played the first phrase, and its repeat just fine and then all the rest of the piece, except the ending, disappeared completely from my mind! I was so rattled by this, which had never happened to me before (at least, not this thoroughly), that I couldn't seem to find my way back into the piece. So I repeated the beginning a few times, improvised some bits, did the ending a few times and slunk offstage to some puzzled clapping. A searing experience, I assure you! I have never since had the slightest urge to ever play that piece again, though I love Torroba's music and play a lot of it. Just not that piece.
Arthur Rubinstein tells a similar story in his autobiography. At a fairly young age he was performing a Brahms piano concerto with the Berlin Philharmonic and it went quite well. But when he went onstage to play an encore, the piece he had ready just vanished from his memory. So he says he improvised something. I hope it was better than my truncated Madroños!
Performers deal with this in different ways. I know of one guitarist who deals with memory lapses by leaping ahead a measure or two. She actually practices this! Of course, the structure of the music is rather maimed in the process, but 80% of the audience won't notice, so, battle won!
I know of another guitarist whose problem with memory lapses got worse and worse so he finally quit playing from memory entirely. Every note in every concert is played from the music. I saw Julian Bream in concert at the Town Hall in New York when he brought the music onstage for a couple of lute pieces--in those days he was doing the first half on lute and the second on guitar. He laid the music on the floor, not using a music stand, and joked that as he was getting older his memory was not what it once was, but his eyesight was getting a lot better. Reading music laying on the floor is pretty difficult and it may have been that he had it there just as a kind of moral support.
I recently heard a concert by a superb guitarist who seems to be having a bit of a problem. Perhaps thirty or forty times in the first half of the concert there were significant memory lapses, covered up, more or less. But the feeling was very much one of the music being just barely held together.
I used to play with violinist Paul Kling who gave the very simple advice: if you think you might have a memory lapse, use the music! He also told me the story of Jascha Heifetz, a violinist of impeccable standards, who once had a horrible memory lapse in a concerto that was so devastating that he chose to end his career rather than risk it again. I don't see anything about this in the Wikipedia article on Heifetz so I don't swear by it, but I suspect it is true.
Suffering a big memory lapse is one of the most devastating things that can happen to a concert musician. Is it related to stage fright generally? Quite possibly, though there are artists who seem to suffer from one and not from the other. It does seem to be the case that one's fears about memory lapses can accumulate over the years. Young artists may actually suffer more from stage fright, because they are less experienced, but less from memory lapses.
In any case, if you see someone experiencing a memory lapse onstage, I am sure that you will be sympathetic!
One of the most difficult composers to perform, in terms of memory, is J. S. Bach, so let's end with some of his music. Here is Andrés Segovia playing the prelude to the first cello suite in the arrangement by Manuel Ponce (with added bass lines):