He says it again later in the same chapter. Then in John's version (Jn 18:11), he's basically saying "how can I not drink the cup my Father has given me?" ... His human side prayed three times for this piece of suffering to be removed. But clearly over time, he knew this suffering was for a reason, for God's greater purpose.
Do you have a situation or circumstance which you want God to remove or fix or change? Sure you can pray for it to get easier, but ultimately just pray for God's will. He will probably change your heart and attitude before he changes your circumstance ... just like he did for Jesus.
