You owe your audience that confident feel so that they are free to enjoy the music rather than worry about you.
In truth most audiences you encounter have come to enjoy your music and your success and are sympathetic. But they would rather offer you admiration than symoathy
A count in is not simply a question of 1-2-3 Go! It is your opportunity to feel the rhythm inside so as to synchronise with it from the exact beginning of the first bar you are playing.
You should hear the performance in your head before you present it to your audience. And of course in the tempo of the count in and the mood and feel you will play your music in.
By having your fingers on the key before you play it becomes a simple matter to give a timely convincing performance. IF your fingers are already in the right place all you have to do is move them up and down! It makes it so easy, so just relax and play!
Of course very often your fingers will have to be moved to a new position via a crossover and you will need an almost instantaneous movement of the whole hand into its new position. That includes the new finger position.
Learn to automatically take account of the key signature or accidentals. It is distracting forr you to have to move twice if you have to alter to cover different black or white notes
This is very important and probably the most difficult part. A lapse in concentration can be caused by many things but just continue to think and prepare ahead.
One of the things you must cultivate as a musician is of course being able to live in two different time zones, thinking of the future while doing justice to the present.
Do not even consider the possibility - remember the instruction "Do not think of a pink rhinocerous" which makes it impossible to think of anything else
But let us say you do play a wrong note. Probably you are the only person who knows that it is a wrong note or that it has occurred - you know the piece, and they do not. So just carry on playing.
Do NOT feel guilty and embarrass your audience by stopping
Give them something else to listen to as you carry on through the piece even with some kind of inside rueful grin. It is not a sin, the worst sin is to upset the continuity by making a Big thing of it.
If you are playing with an orchestra and you all land up in a virtual musical heap on the floor that is another thing of course! Then you will alll need to stop and pick up the pieces.
Confidence comes from a sound technique too
A background knowledge of how music is put together
Everyone knows practising helps, but how do you do it effectively
Playing from chords and recognising them in music