The craft of self-critique is an important one for any musician trying to get to do anything better.
You can’t rely on a teacher / coach / supervisor to do everything; if you’re me and a lot of other people in similar positions, you can’t afford a teacher to do anything. But even if you are having lessons, there’s little point doing any practice in between them if you can’t tell which bits of a piece you play well, which you play badly.
At the more independent level, it’s important to be able to tell whether you’re actually making progress with something. If it’s getting worse, then probably time for some serious analysis of why that is. If you’re hitting a brick wall, then as an amateur there might be room to just decide the piece / movement / passage is too hard; if getting paid or just more committed, then failing to improve calls for a cast round for other ways of attacking the issue (is there a better fingering? should I revise the bowing? does thinking of this song as in G flat rather than F sharp make it possible to create a nice fiddle line? do I need to slow this passage right down and then gradually build it up to speed? etc.).
But then there’s also the question of how good is good enough. All right, I ‘can play it’. But is it at the right speed? Is it in tune? Am I producing enough (or little enough) volume for ensemble balance, venue, audience in the context I expect at the (next) performance? Is the tone good enough, and judged right?
All of which are valid points to listen to yourself for, and good areas to focus on. But, a lot of pro contexts provide seriously limited practice and rehearsal time, meaning that prioritisation is necessary. That bit can be a bit sloppy, it’s in the middle of a full orchestral tutti and really just texture / effect anyway. That’s a solo so I have to get it right; then again no point worrying too much about tone because amplification will distort it anyway. This way you can create enough time to sort out the biggest problems, hopefully managing to deal with all the ones the client / audience would be likely to care about (but see the previous post on being ready!).
But then of course the process of self-critique can become itself enveloping, even if you’ve managed to avoid the punishing perfectionism that will leave you incapable of ever being content with your performance (and that’s a high road to no professional career if ever there was one. How are you going to hunt down work and promote yourself hyperactively if you think every note you play is dire?). If you’re really listening to (perhaps watching as well) pitch, timing, phrasing, volume, tone, accuracy to the sheet music (or the original if you’re in a non-reading covers band), and various aspects of technique that aren’t directly evident in the sound but in which bad habits could hold you back, then you probably have significantly damaged your concentration on actually playing, inevitably worsening anything that’s difficult – and if it’s easy for you, why are you practising it? Unless perhaps to get a finer polish on subtleties, in which case what you’re practising isn’t easy for you and we’re back where we were.
In practice sessions or rehearsals, the impact of overselfanalysing may just be a knock to confidence – though it will slow you down since you’ll underestimate your ability / mastery of specific repertoire and techniques because you’re too busy estimating to do the best you can.
But I’m a great believer that you get better at what you practise, and you tend to perform how you mostly play. So if you spend much much more time practising by yourself, very critically, than performing (and that’s not a dead letter – I probably spend almost as much time, even now, in rehearsals and performances as practising alone), then it’s unlikely you’ll magically switch off all that self-criticism when you get up to actually perform for real.
There is a legitimate space, particularly if and insofar as you repeat repertoire fairly heavily and/or perform repeatedly with the same people, for seeing a performance as always partly a rehearsal for the next one. BUT the space for such analysis and constructive critique is retrospective (probably the one real justification for the otherwise somewhat redundant and effort-wasting practice of rough-recording all or many of your gigs). ANY process of analysing a performance while you’re giving it is simply going to make the performance less focused, and just as importantly less audience-directed and so less engaging.
There’s an underlying assumption in this post that if you’re serious about music, you will be capable of self-critique, of telling yourself in an empty room that something isn’t good enough yet, or is in one respect but not another, or will be soon but isn’t yet – otherwise you never get to the standard of music-making to be able to be really serious about it. But, it needs to be an ability you can switch off. Otherwise you’ll only ever give your best in your soundproofed practice room, if then; if you ever reach a live concert platform it’ll be a disappointment to all concerned.