How do you unlearn a mistake you have practised into place?

Your diagnosis is right and it points at the fix. The problem is not the difficult bar — you can play that. The problem is the junction, and you have never practised the junction as its own thing.

The technique is sometimes called building a bridge, and it is the single most useful practice method there is:

1. Take the last note or two of the earlier bar and the first note or two of the later bar. Nothing else. Two or three notes total.
2. Play just that fragment, very slowly, until it is completely secure.
3. Extend backwards by one note. Secure it.
4. Extend forwards by one note. Secure it.
5. Keep growing outward until the fragment covers both bars and joins the surrounding music.

The key insight is that you are practising the transition itself as a unit, rather than practising two things you already know and hoping the join looks after itself. That join is where the actual difficulty lives and it has never had any attention.

The other half is the one people find hardest to accept: slow enough that the mistake cannot happen.

Not slow-ish. Slow enough that you have time to see the next note, place the hand deliberately, and play it correctly with complete certainty. If a mistake occurs, you were still too fast.

The reason this matters is that repetition strengthens whatever you actually did. Practising at a speed where you fail half the time reinforces both versions, and the wrong one has a head start. Every correct slow repetition is a vote for the right version, and votes only count if they are correct.

Then increase the tempo in genuinely small steps, and drop back the moment an error appears. Most people go up in steps far too large and spend weeks at a speed that is producing errors.

One idea worth knowing from wind and brass pedagogy that transfers directly: think about the sound you want rather than about the fingers.

The teaching, associated with a well-known orchestral player and teacher, is that trying to control the mechanics consciously interferes with a motor skill that operates better below conscious attention. What you can control is a clear mental picture of the intended result, and the body organises itself towards it.

In practice, for your bar: before playing, hear the transition in your head exactly as you want it — the timing, the tone, the shape of the phrase. Then play towards that image rather than thinking about which finger goes where.

This sounds mystical and is not. It is the difference between micromanaging a movement and giving it a target, and for many players it unblocks a passage that technical practice has stalled on.

One more thing that often turns out to be the actual cause: check the fingering at that junction.

A transition that fails consistently, in the same way, in an otherwise easy piece, very often has a fingering problem — the hand arrives in a position from which the next notes are awkward, and no amount of practice fixes an arrangement that requires the hand to be in two places.

Work backwards from the note you must reach and see what hand position it needs, then choose the fingering in the previous bar that puts you there. Sometimes the fix is one changed finger three notes earlier, and the difficulty evaporates entirely.