Piano Forum

The tempo of the passage is such that it will not be very noticeable if you do not hold down the key with your thumb.  Also, the thumb note is not a focal point in that passage. 

Think about the performance tempo that you want and how many notes of that trill will fit.  Practising the passage at a slow tempo gives more time for notes in a trill, but might give a magnified appearance of difficulty because you may be trying to fit more notes of that trill as compared to what is actually needed in performance tempo. 

At tempo it might be:
C, D, C, B, C
or
C, D, C, D, C, B, C


For exercises:

Play a tremolo C-D (major 9th). 

Play C-D trill with 4-5, extending the thumb over C but not holding. Don't make the thumb static, let it wave over the keys, as necessary to balance the 4-5 trill.  Try to use a similar wrist movement as the tremolo.  Aim to have a similar sensation in the thumb as the tremolo, even though the thumb is not playing any notes. 

Alternate between the tremolo and the trill without break.  Try to use a similar movement in the hand.

Keeping in mind that sensation of tremolo in the hand, play the trill while holding down the C with the thumb.  Aim to have the thumb feel like it is playing a tremolo, even though it is just holding a key down.