Any tips on bridging the gap - amateurs practice till they get it right vs professionals practice till they can't get it wrong.
At a minimum, you had better amend that to "till they
consistently get it right", not just until they get it right once by chance. I want to see something right on the first two tries, or three times in a row after previous stumbles, before I move on to the next segment / move the metronome to the next notch.
Getting a piece ready to take out in public involves two very different phases, at least for me.
Learning a new song, it pays to be very strict with yourself. Start very slow, get everything right. I let myself miss
one note and keep going; if I miss two in the same phrase, stop, back off, try again. If you barge through a passage only hitting 90% of the notes over and over, you will get really good at playing those 10%-wrong notes. Fix it before you burn an error into your brain with repetition. It is really hard to fix a note you learned wrong and played wrong a hundred times already.
Along those same lines -- it's also easier to be expressive if you plan from the beginning to be expressive and practice that way from the start, than if you add in the dynamics etc later.
Once you have all the notes, then it is time to work on speed, and use that evil metronome as much as you can stand it. If you feel the metronome is "stifling your expression" have it beat half-notes instead of quarter-notes, so you can borrow a bit of time from one beat to the next, but really, it can count better than you can, and you'll be shocked at just how
bad you are at keeping time on your own if you record yourself and then watch the playback.
Phase two is quite different. For a piece
you've already learned, you need to work on recovering gracefully from errors without stumbling. (I had a lifetime of practice with this, playing 2nd violin in a big orchestra... it was all about not getting lost and not making a mistake anyone could hear, not about playing every note.)
Now you don't let yourself stop, and see how well you can get back on track when something goes amiss.
You can help yourself if you make sure to start in different places when you're in phase one, so that you have some experience jumping back into the piece anywhere you need to.
Nothing ever goes amiss? Bring in some distractions.
Play in a different room of the house than your usual practice room. Everything will sound a bit "off" and you won't be so sure of yourself. Have your dog in the room with you and see if you can keep playing when he barks and whines at you. Take it out on the porch instead of indoors, and try to keep playing when a noisy truck goes past. Go outside when it's a little bit too cold or a little bit too hot, and make sure you can play under those conditions. ("Too hot" is a tough one for me -- I just don't want to do ANYthing if it's over about 75F, and that is real bad news if you have aspirations to busk on a street corner, or play at a summer music festival outdoors.)