A (VERY) NEW PHILO APOCRYPHON is noted by Torrey Seland.

UPDATE: Stephen Goranson e-mails:

"Be kind, for everyone you meet is fighting a hard battle."
This quotation (or a variant) is often attributed either to Philo or Plato, though I've never seen a specific citation.

For what it's worth, here's the closest thing I found by searching JSTOR. In The Biblical World (retitled the next year as The Journal of Religion) v. 54 n. 6 (1920) page 606 Ozora S. Davis, in an article on preaching quoted II Peter 1:57 and then commented on each phrase, including: "_brotherly kindness_--Everyone is fighting a hard battle."