A Response from John Stott:
…we need to distinguish between the general and the particular will of God. Since God has revealed his general will for all his people in Scripture (e.g. that we should control ourselves and become like Christ), we should indeed pray with definiteness and assurance about these things. But God’s particular will for each of us (e.g. regarding a life work and a life partner) has not been revealed in Scripture, so that, in praying for guidance, it is right to add ‘by God’s will’. If Jesus himself did this in the garden of Gethsemane (‘Not my will, but yours be done’), and if Paul did it twice in his letter to the Romans, we should do it too. It is not unbelief, but a proper humility.
—The Message of Romans, 389-390