don't count your chickens before they hatch

don't count your chickens before they hatch (don't get your expectations up too high, you might be disappointed.) — цыплят по осени считают; не говори гоп, пока не перепрыгнешь; не радуйся раньше времени; не дели шкуру неубитого медведя
Example Dialogue:

A: "I am so excited! My friends said my audition was the best! I am certain to get the part."
B: "Well, sweetie, don't count your chickens before they hatch."

Don't haloo until you are out of woods — не радуйся раньше времени; не говори гоп
There's many a slip betwixt the cup and the lip — не говори гоп, пока не перепрыгнешь

It's not over till it's over. - не радуйся раньше времени, цыплят по осени считают.

see also
[it ain't over until the fat lady sings]
[jump the gun]
[забегать вперед]