So the idea is to use much force on those thirds to make them more solid and then use less force in performance? I practiced today and found that slow simple practicing with a metronome, and practicing the thirds in all dynamic levels from pp to ff helped and I think I'll be fine. Thanks to all who spoke.
By the way, Bernhard, do you have a link to a post about overcoming "difficult passages"?
Strictly speaking there are no difficult piano pieces/ sections. However there are very complex piano pieces/sections. The way to deal with complexity is to simplify it. The main ways to do that are:
i. Practise in small sections (some times two notes only)
ii. Outline the piece/section.
iii. Deconstruct the piece/section in layers and then learn each layer in isolation, then combine and add the layers (e.g. in counterpoint music learn each voice separately; in motif based music, learn only the motif and its occurrences/variations, then add the non-motif notes, etc.)
iv. Practise hands separate.
Here are some threads that deal with some of these aspects in more detail:
https://pianoforum.net/smf/index.php/topic,5298.msg50376.html#msg50376(alternative to the chord trick – Rhythm variations - repeated note-groups – starting with the difficult bars – how to break down a piece in sessions – ways to tackle speed that do not involve the chord trick – Bernhard: A plagiarist?)
https://pianoforum.net/smf/index.php/topic,1867.msg14268.html#msg14268(Getting technique from pieces – several important tricks: hand memory, dropping notes, repeated note-groups)
https://www.pianoforum.net/smf/index.php/topic,9285.msg94312.html#msg94312(outlining: example – with score – Chopin etude op. 25 no. 1)
https://www.pianoforum.net/smf/index.php/topic,7810.msg80415.html#msg80415(How to organise the breaking down of a piece over several days. Examples: CPE Bach Fantasia, Blow Sarabande, Chopin Cantabile; Chopin Scherzo no. 2)
https://www.pianostreet.com/smf/index.php/topic,7341.msg114168.html#msg114168(repeated note-groups for difficult passages – correct technique is never uncomfortable – rotation as the solution to 5th finger weakness – criticism to misguided technical exercises – trusting the unconscious)
https://www.pianoforum.net/smf/index.php/topic,7246.msg72307.html#msg72307(how to outline invention 14)
https://pianoforum.net/smf/index.php/topic,3561.msg31700.html#msg31700(Questions about Bernhard’s method – Bernhard’s answers – mostly about the 7 X 20 principle, how do you know when you mastered a section, when to use the methods, and when they are not necessary – investigating the reasons for difficult)
https://pianoforum.net/smf/index.php/topic,4689.msg44184.html#msg44184(Bernhard answers questions and elucidates further about: 20 minutes – practice starts when you get it right – definition of mastery : learned – mastered – omniscience – Aim for easy – final speed in practice must be faster than performance speed – Example: Chopin Op. 10 no. 2 – outline – repeated note groups – HS x HT)
https://pianoforum.net/smf/index.php/topic,4710.msg44538.html#msg44538(Bernhard explains once more about 7 x 20 minutes – Progress is the ultimate decider – How to break a piece in practice sessions – Example: Satie gymnopedie – importance of planning – aim at 100 pieces per year – Example: Bach Cm WTC 2 -)
https://pianoforum.net/smf/index.php/topic,4750.msg45125.html#msg45125(more details: learned – mastered –omniscience – why repertory must be paramount – how to work on 20 pieces per month – a case for easy repertory – importance of discipline and of having a plan – analogy of mastering a piece and making wine – musicality is ultimately good taste – Example: Beethoven op. 49 no. 2- A list of progressive repertory to lead to Rach prelude op. 32 no. 5 – mastery is when it is easy)
https://pianoforum.net/smf/index.php/topic,4797.msg45744.html#msg45744(No skipped steps – Bernhard enlightens further and tells the usual places where students go wrong – Ht x HS)
https://pianoforum.net/smf/index.php/topic,4858.msg46087.html#msg46087(Paul’s report on B’s method. Feedback from Bernhard including: HS x HT – Example: Lecuona’s malaguena – 7x20 – need to adjust and adapt – repeated note-groups – importance of HS – hand memory – 7 items only in consciousness – playing in automatic pilot - )
https://pianoforum.net/smf/index.php/topic,4059.msg37072.html#msg37072(Does there come a time when piano learning will not be so difficult – The problem: a good teacher, enough practice and yet very slow progress – approach maybe everything: practice to make it easy. Guardian link to Alan Rustrbridge article – summary of PPI - alternate hands)
https://pianoforum.net/smf/index.php/topic,4004.msg47440.html#msg47440(3 correct X wrong attitudes when practising)
https://pianoforum.net/smf/index.php/topic,4123.msg37829.html#msg37829(How to investigate the best movement pattern: Example Scarlatti sonata K70 – How to work out the best fingering. Example: CPE Bach Allegro in A – Slow x slow motion practice – HS x HT – practising for only 5 – 10 minutes)
https://www.pianoforum.net/smf/index.php/topic,7664.msg77057.html#msg77057(How to break a piece in sessions – Example: Invention no. 1).
I hope this helps .

Best wishes,
Bernhard.