“You don’t know what you are asking,” Jesus said.  “Can you drink the cup I drink or be baptized with the baptism I am baptized with?” – Mark 10:38
James and John wanted to be as close as they could with Jesus no matter what cost.  What Jesus was referring to in regards to drinking the cup was that God the Father is going to pour out His Wrath onto Jesus in redemption for our sins.
When we petition to our Heavenly Father requests to be a better father, husband, or ask for more opportunities for faith, we need to make sure we do not ask lightly.  We need to understand at what cost it will take, otherwise, it will be too overwhelming.  However, do not withhold your requests as long as they come from a selfless heart.  Jesus will see you through it until completion.