The ideas presented here are based on the same principle as presented in How to avoid stopping when I play a wrong note on the piano. I recommend reading that answer in addition to this one, possibly before reading this one.
The core of the problem
Problems like this one are often attributed to muscle memory, and physical solutions are the typical prescription. However, in the absence of a genuine physical limitation (for example, an injury that somehow prevents accurately playing certain notes), there is near always a deeper problem that, when solved, allows the muscle memory to take care of itself or at least be much easier to fix. That deeper problem lies in one's emotional understanding of the music. Emotions drive the movements we make to play the piano. (If not, then we're just robots mechanically following instructions.) Confusing two notes/measures/sections after having learned the mechanics of them means we associate or confuse them on a felt level.
To most efficiently fix the problem requires understanding the "meaning", "feel", "story", "picture", "color", or other emotional association(s) of the two measures. What makes the two measures feel different from each other?
CAVEAT: What follows is based on my interpretation of the piece. Yours may be different. The goal here is to demonstrate an approach, not to dictate an interpretation. The key is that you practice hearing/feeling the interpretation you find best fits for you; this is how you will fix the problem.
How do we find the difference between measures 7 and 11?
What is the larger context?
The whole piece
The title of the piece is "Lamentation for a Lost Life", and the feel of the music bears this this out: slow, sad, nostalgic, but also, in seeming contradiction, dance-like.
Within the piece, we can hear three main sections: a dance-like, nostalgic melody that feels somehow incomplete (measures 3–18); a section that feels similar, but very hollowed out (measures 19–34); and a return to the first section (measures 35–50). It's worth noting that while all three section share the identical left hand part, the middle section right hand is a stripped down ("hollowed out") variation of the melody of the first section.
The emotional progression of the piece is: bittersweet, hollow, bittersweet.
The first section
Since measures 7 and 11 are both part of the first, bittersweet section, we'll focus on that.
As with the piece as a whole, we can hear three sub-sections: a sad little dance that trails off (measures 3–6); a slightly happier repetition of the little dance which again trails off — perhaps an attempt to feel a greater presence of whomever was lost, but ending in just feeling the loss again (measures 7–10), and an attempt to revive the dance (feel the presence of the lost life) that keeps sinking lower and finally peters out (in a sense of loss) (measures 11–18).
The first section is sad, tries to be a bit happier by feeling the presence of the lost life, and then tries even harder to be happier (feel more presence), failing every time (experiencing loss).
What does this mean for measure 7?
Measure 7 is the beginning of a happier attempt at the little dance. Since it's trying for "happier", I would add some energy. I would do this by making measure 7 just a bit louder than measure 3 (note the shift from minor [measure 3] to major [measure 7]). I also might press the tempo very, very slightly in measure 7. Speeding up and playing louder both add energy — greater musical "presence" — and happiness (felt presence) is more energetic than sadness (loss).
Measure 7 is has a bit more energy/presence than measure 3.
What does this mean for measure 11?
Measure 11 not only kicks off another attempt at a happier memory, but it's a more earnest attempt: 8 measures instead of 4. And it tries to use only the dance part (presence) of the initial musical idea (measures 3-4 and 7-8), avoiding the "trailing off" (absence/loss) part (measure 5-6 and 9–10). The measure 11–18 section repeats the dance idea three times, but each time lower and lower in pitch. There's a tension between the effort of trying again and again, and the "defeat" of sinking lower and lower.
Measure 11 needs more energy than measure 7, because its the beginning of the most substantial emotional effort so far.
What is the musical difference between measures 7 and 11?
Measure 7 needs enough energy to support a four-measure idea that starts off a little bit happier or warmer than measure 3, but ends by fading away.
Measure 11 needs enough energy to sustain an eighth-measure idea that makes a substantial effort at happiness or warmth.
I would play measure 7 a slight bit louder than measure 3, and measure 11 would be louder than measure 7. I would also consider subtly pushing the tempo in measures 7 and 11 to add some momentum at the beginning of their respective phrases. Again, measure 11 needs a little more momentum than measure 7.
How to practice this?
- Play measure 7 with the emotion you want behind it. If this takes more than 3–5 tries, you likely need to look deeper for what else might be mission at the emotional/interpretive level.
- Play measure 11 with the emotion you want behind it. If this takes more than 3–5 tries, you likely need to look deeper for what else might be mission at the emotional/interpretive level.
- Play measure 7 and then measure 11 (back to back, but with a pause in between), making sure you can feel/hear the difference between them. If this takes more than 3–5 tries, ....
- Play the first phrase with the overall musical "shape" you want. If this....
Invariably, fixing problems in this way "creates" other problems. That is, deepening one's emotional understanding of a note, a chord, a measure, etc. will automatically reveal other notes/chords/measures/... that haven't yet achieved the same depth of understanding.