When I try, I fail. When I trust, He succeeds. - Anon
I think this concept, and the reality, that 'I can't do it' is one of the hardest to keep ahold of though it is entirely, 100% true. Because I can't do it. Only the LORD can cause His will to come about. Only the LORD can see His will accomplished. I, myself, truly can do nothing in my own strength to ensure His will is done. I can't even ensure my next breath.
God is not only sovereign but He is the force through which all things pleasing to Him occur. He opens hearts and shuts doors. He causes nations to rise and fall. He sends rain for harvest and causes the ground to spring forth. He commands our every step and causes us to prosper. He brings about trials to test our faith and demonstrates our utter dependency upon Him.
Truly, we can do nothing in our own strength. Only when we depend upon the LORD will doors open, paths be made clear, and thoughts be established. Only when we place our trust in Him and keep our eyes ever upon His face will we see Him work on our behalf. Faith and trust must absolutely be placed in our LORD and Savior for the big and little things of life - for He alone will do the work and fulfill His will for our good and His glory.