I agree. But how do you prepare mentally? I am not asking about technical prep, as that is not the problem with me usually.
1. Practice the situation of having to perform. Use any possible opportunity to play your pieces to an audience. Friends, family, lovers, colleagues, whoever. An audience of one will suffice. You may play all your pieces, or just one, or just a movement of a sonata: whatever your audience has time for. If you don't think the pieces are quite ready, play them anyway: explain to your audience that this is the case, that you may make mistakes or even be obliged to stop, but you want to use the opportunity to test yourself. The mere fact that somebody is actively listening puts you into a "performance" situation: this situation needs practice, just like technique does.
2. Visualisation. Set aside a time for this when you can be undisturbed and play through your complete programme. Alone with your piano, visualise the recital, then go through the whole recital process. Make the visualisation as vivid as possible: start by imagining where you are when you warm up. Do the sort of warm-up you would normally do just before the concert. If it won't be possible to warm up just before the concert, leave as much time between the visualised "warm-up" and "concert" as there will be for the real event.
Now leave the piano and imagine the stage with the piano on it. Try to see all the details in your mind: where is the audience seated, how many of them are there, are there people you know, how far do you have to walk to get to the piano... If you start feeling nervous: great, you have good powers of visualisation.
Go to the piano, bow to the imaginary audience, sit down and start playing. All the time you are playing, keep focused on that imaginary hall with the audience. If you make a mistake, try to cover it up as elegantly as possible but
keep going! Impose exactly the rules that the live concert situation will impose. (Did you know that Franz Liszt was renowned for turning mistakes into "features"? He would sometimes carelessly hit a very obvious wrong note, but he would never look disconcerted: he would pause a moment on the jarring harmony, then with a sly smile he would improvise some figure based on this harmony, finally getting back to the point where he would find his way back into the piece and go on as if this was all intended).
If you go through this "mockup" of the recital a few times, you will be well prepared for the real thing.