In a recent chamber music dress rehearsal, I had a surprising lapse on the opening phrase.

From Reinecke, Trio for Clarinet, Horn, and Piano, Op. 274 (Horn in F)
The opening is horn alone, which is enough to trigger an extra burst of nerves. I had no trouble with it in previous rehearsals, though. In the dress rehearsal, after I landed on the C5 dotted half note, everything that followed a jumbled mess. Nothing came out right. It was an embarrassing way to begin the piece.
In the spirit of a dress rehearsal, we continued to play as if nothing had happened. Later, I replayed it in my head. What happened? What was I feeling and doing in my body and mind? How could I make sure it doesn’t happen again in the performance?
I realized that the opening pickup (C4 and the octave slur to C5), had been occupying all of my attention. I was nervous about them. They are the first notes of the piece, with no one else playing, and they needed to be solid.
With my mental replay of the dress rehearsal, I realized that, after landing on the C5, my mind checked out for just a moment. I lost track of what was coming next, and my fingers got fumbled up, and it was a train wreck.
Lately, I’ve gotten a little better about not beating myself up over such things and asking myself, instead, if there is something I can do to minimize the chance of a repeat.
First, I memorized the short phrase so that I could mentally rehearse it, including the fingering changes. I played it my mind repeatedly, with my fingers barely moving against my leg with the note changes.
Second, I decided that, in the performance, I would pause for a few seconds before beginning, just long enough to look (and really see) the entire opening phrase, hear it in my mind, and let my fingers lightly press against the valve levers with the fingering changes. I wrote a note in the part to remind myself to do that.
In the performance, the opening was fine. Maybe it would have been fine if I hadn’t done those things. I think, though, that mental rehearsal of the entire opening phrase, and focusing on the entire opening phrase and not just the first couple of notes, is a good approach that is worth keeping.