Antonym to "Don't Count Your Chickens Before They Hatch"

Solution 1:

I could try to come up with an antonym to my own interpretation of Don't count your chickens before they hatch.

But you have provided your own interpretation:

don't assume the optimistic scenario will happen before it does

Or, don't gamble everything on something working out the way you hope it will. (Don't get ahead of yourself.)

For an antonym, what you're looking for, again in your own words, is an idiom that means:

don't assume the worst until it actually occurs

I take this to mean that, everything being equal, you should not be defeatist about possible outcomes. Or that you shouldn't let the possibility of a poor result stop you.

In that spirit, what come to mind are the following:

Nothing ventured, nothing gained.

[Merriam-Webster]

—used to say that it is worth trying to do something because one might succeed even though success is not certain

Never say die.

[Merriam-Webster]

—used to encourage someone to continue something or to remain hopeful
// It doesn't look good for the team, but never say die. They could pull off a miracle.

Solution 2:

"Don't look a gift horse in the mouth" would be an appropriate antonym to "counting one's chickens before they are hatched".

One would examine the teeth of a horse to determine its age. If one was gifted a horse, it would be quite pessimistic, and prematurely so, to examine its mouth before accepting it.

Of course, the nature of idioms is that there probably isn't an exact satisfactory opposite to any given one. There will always be shades of meaning that aren't possible to mirror.

The idiom "don't count your chickens" certainly is not about giving a gift, or being grateful, and the idiom "don't look a gift horse" is likewise not about the anticipation of personal profit. Nevertheless, these two idioms do mirror each other on the axis of optimism/pessimism in anticipation of a future change in fortune, whether it is the addition of a single horse or a number of chickens.

Solution 3:

Don't cross that bridge till you come to it.

Do not needlessly worry yourself over concerns, problems, or difficulties that lie in the future.

 I'm not sure why you're so concerned about how to write a thesis for your degree—it's over a year away, so don't cross that bridge until you come to it! 

I know you're worried about the mortgage payment in January, but don't cross that bridge till you come to it.

(idioms.thefreedictionary.com)

Solution 4:

Don't borrow trouble from tomorrow.

This proverb, in various forms, has been around for nearly 200 years. See Google books. It still seems to be in use.

Some of the early citations treat it as a paraphrase of a biblical passage (Matthew 6:34):

Take therefore no thought for the morrow: for the morrow shall take thought for the things of itself. Sufficient unto the day is the evil thereof.

It possibly might be where the idiom "borrow trouble" comes from, although "borrow trouble" has now acquired a different meaning; from Oxford Dictionary Online:

Borrow trouble, North American: Take needless action that may have detrimental effects.