“Never discourage anyone who continually makes progress, no matter how slow.” is a phrase popularly attributed to Plato on the internet, but after extensive googling, I can't see any reference to any of his works that contain it. Does it appear in his writings, or is there anything similar in his writings? and, if so where does it appear?
No one should be discouraged, Theaetetus, who can make constant progress, even though it be slow.
And the Jowett translation at Project Gutenberg translates it as:
Any one, Theaetetus, who is able to advance even a little ought to be of good cheer
And in the Nicholas P. White translation in Hackett Publishing's Plato: Complete Works, it's translated on p. 285 as:
Even if you can only make a little progress, Theaetetus, you should cheer up.
According to the perseus.tufts.edu page, this line is from location 261b in the text, the corresponding Greek text can be seen here.