The closer we walk with the God, the more everything gets directed by Him. He starts molding us and fashioning us according to a specific design He had for us from the beginning. Once this process is set in motion, it is paramount that we submit to what He is doing rather than rebel against it. It can be a painful work, but as Paul said, in due time it "reaps the peaceable fruit of righteousness" in us (Hebrews 12:11).
Who knows whether tomorrow will include pains and trials, or blessings and joys? We don't know what tomorrow or the next day will bring, but we can trust that He already saw it coming, and planned on it ultimately molding us into the person He wants us to be. So the godly man says, "If it be His will, I will do such and such today or tomorrow," because he knows his life is in God's hands, and He is fashioning him after a particular design.
A fuller quotation of the Jeremiah passage implies that even if we don't turn out the way we were supposed to on the first go around (maybe due to our stubbornness, or "knots" if you will), He keeps trying again until we finally turn out right.