Don't count your chickens before they are hatched
Don't be hasty in evaluating one's assets.
Many of the proverbial words of advice that have lasted the test of time begin with 'don't'. We are warned not to 'keep a dog and bark ourselves', 'look a gift horse in the mouth', 'change horses in mid-stream' etc. 'Don't count your chickens' is one of the oldest, and possibly the wisest, of these. The thought was recorded in print by Thomas Howell in New Sonnets and pretty Pamphlets, 1570:
Counte not thy Chickens that vnhatched be,
Waye wordes as winde, till thou finde certaintee
Samuel Butler continued the pleasing rhyming in his expression of the proverbial advice, in the narrative poem Hudibras, 1664:
To swallow gudgeons ere they're catch'd,
And count their chickens ere they're hatched.