Knowns :
-Repertoire/Material
-Who you are
-What you are thinking
-How much work has been put in
-What kind of results you have had in practice
-Your own body and how it relates to the repertoire
-Your own body and how to use it at the piano
-Your own experiences (how much, how little)
-Your own desires
-Can get yourself out of trouble/can cope with mistakes
-You will (most likely) survive
-How to relate with people (in a number of "different" ways : socially, artisticly, spiritually, authoritatively, etc)
Unknowns :
-Exactly who is in the audience (you do have some control over this)
-Exactly what they are thinking (here again, you can have *some* sense about what your context is for performing and what is being expected of you)
-If people will like it/you or not (most people, on a fundamental level,
want to like other people)
-The piano*
-The venue* (acoustics, temperature, general feel, etc.)
-What kind of mistake will be made (though there is quite a bit of control here, too)
*These may actually be/turn into "knowns" - and you may have some control over that.
There are probably a number of things that can be added to my lists above, but as you can see, there are actually very few items that are truly unknown and completely out of our control -- and that mostly has to do with other people and what they are/might be thinking (though, there is a caveat here, too

). Also, there are items in the "known" list that may not be truly apparent at different times of a person's life -- like who you are, for example, or what your desires are (these things can actually be cause for more nerves than a person might think). I have spent many a pre-performance in a kind of deep, subconscious state of wondering who I truly am, what was I doing there, where was I (?), why was I really there (?) ... these sorts of things. I have had many nerves, if not the majority of them (and my most debilitating ones) simply because of feeling a bit out of place. This kind of state, coupled with a number of other "unknowns" that
could potentially be knowns --- like knowing the repertoire really well --- can make for a pretty scary experience.
Now, an interesting thing about the "knowns" category is that not *everything* has to be perfectly in place in order to have a successful performance. I think that ultimately, knowing the repertoire and knowing that you know it, are top priority in terms of a person's level of comfort goes in the performance itself. However, even if there are questions for you about how well you truly know it, but if you know yourself really well and if you know that you can pull yourself out of mistakes, you can definitely artistically, creatively, and musically survive a performance (this will depend a bit on the audience, too). You can probably sense that having as many things as possible be "known," and as few things as possible be "unkowns," the better.
Performance itself is an art, but it's something that can be practiced in every moment of our lives. This is a reason that actors who are very dedicated to their work will often enter a mindframe that the whole world is a stage ! Who we are as a performer is not truly anybody else than who we are as a person in everyday life -- at least there is *a lot* that we can pull on when it comes time to perform.
Some very practical advice is that performing must be practiced. For every level of performance we are aiming for, there are levels of preparation that can give us steps in the right direction. For example, if I am aiming to play for my family reunion, it will start with me practicing and performing for myself. Then I may turn on a recording device. I may play for my doggy. Then I may play for a trusted friend, or a few trusted friends. "Visualization" can be utilized before every one of these events (including playing for oneself).
Finally, I have found that playing the same repertoire many times -- and in various scenarios (ideally a *very* long string of performances (100 or so)) -- is one of the best ways to gain confidence as a performer.