His will be done?

What is the point of praying if God already has a plan in mind for us? Why interject our own desires into prayers of petitions if we are expected to pray for His will to be done? If I pray for something so earnestly all my life and it is not what God has in mind for me, then I have wasted thousands of prayers in vain. What I desire may or may not be what God has in mind for me, I have no way of knowing. So I ask, why bother?