“Drink the cup before you” is the biblical version of our modern phrase “play the hand you’re dealt.” It means there isn’t much you can do about the circumstances of life, so you should just play along with them. But every once in a while there comes an opportunity to change your circumstances. Peter had an opportunity like that where he had to decide between drinking his cup and fighting for a new one. Peter had to be reconditioned in the way he understood God's will - as does everyone who wants to follow Jesus.