The situation: Your best shot is missing, and you are losing because of this. Should you keep using it, or abandon it? It takes years of tournament experience and hard thinking before a player can consistently make a sound judgment in a situation like this as to whether to change his strategy, or keep using the shot that is missing in order to get it going again. There are three possible reasons why you are missing your best shot: your opponent is doing something to throw you off, you are nervous or unfocused, or for some other reason you are simply off.
Many players, even at the advanced levels, do not recognize when an opponent is doing something that is throwing them off. Those that don't recognize these strategies often talk and think strategy quite a bit - but only from their point of view, forgetting to take the opponent's strategy into account. Ideally, you neutralize your opponent's strategy by dominating with your own - but to do so, you need to know what the opponent is doing, or is capable of doing. So the first thing to do is figure out whether you are missing because you are really off, or because your opponent is doing something to throw you off. If the latter, then you have to find a way to counter it.
Psychologically, you have to learn to be calm and focused during a match. That's mostly separate from the tactical side. If you do get nervous or lose focus, that's a good time to take a one-minute timeout, or at least take your time between points to get yourself together. Nervousness or lack of focus are the most common reason for a player's best shot to start missing. Never play a point until you are calm and focused.
Finally, there are those times when, for inexplicable reasons, you are simply "off," and your best shot keeps missing. Usually it's one of the two previous reasons given, though you might not realize it. But it also might be because you are out of practice, your muscles are tight, equipment problems, problems with the playing conditions, or who knows what else.
In all of these cases, this is where the judgment of years of play can pay off as you judge whether to keep using the shot in the hopes that it will come back, or switch to other shots and strategies.
A good general rule is that you have to get your best shot going in any competitive match or you'll probably lose. Most often you should go down with your best shot, since you are also likely to end up winning with it. But if you see a way to win without your erratic best shot - usually by taking away your opponent's best shot, so both of you are going with your "B" games - then you might want to take it.