Like George said, the musical education wisdom is indeed to practice slowly first, then work up to the final speed incrementally. Also, when there are a few difficult spots, practice them separately until you get them right. Dont just repeat the whole song over and over when there are just a few trouble spots.
But of course, you could learn all this in traditional music education... so here is some unofficial advice for the casual performer (actually also done by professionals a lot): learn how to wing it. You can either study a simplification or just try an approximation of what the score says when you think the audience will not notice. Second, you need to play with a poker face, pretending that what you play is exactly what it should be. When you see some music arrangements made by accordion players you can already see that they anticipate the show element of pretending.
Two examples: In one solo I have to play a (very fast) run going up E D C# D E F# A C# E D C# D E F# A C# D (on C system CBA). The way my hand works I often hit A# (Bb) instead of A at least once by mistake, playing E D C# D E F# A C# E D C# D E F# A# C# D. Sure this is wrong but nobody will notice. (The whole run takes under 2 seconds.) Another example is when the composer or arranger already anticipates that show is more important than notes. Consider the following run down: G F E D C# B Bb Ab G F E D C# B Bb Ab G F E D C#. Try this on your PA ;-) The arranger actually anticipated the use of CBA and the whole run goes chromatically over just two rows of a CBA. The famous very difficult piece Rondo Capriccioso by Soljetarov is full of such 2-row runs and makes life extremely difficult for PA players and a lot easier (still difficult) for CBA players. Its all for the show.
How do the tricks of simplification plus poker face in case of errors work in practice? Watch this video by Alexander Skliarov. Can you tell where he plays a plain wrong note? Can you tell where the run he performs is a simplification of what the composer (Novikov) wrote? Probably not.
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At 1:28 he plays a run with only half the number of notes of what the composer wrote (the run is simplified to being just chromatic over 2 rows) and at 3:49 he plays Gb F Bb... instead of Gb Ab Bb (F is the button next to Ab). Absolute poker face there. (There are more slight mistakes hidden in a mass of notes, but these two are in plain sight, not caused by technical difficulty.) Despite these mistakes it is still one of my all-time favorites. Alexander is just such a brilliant player, the composition by Novikov is brilliant and the sound of the AKKO bayan is exquisite...