"Therefore do not be anxious about tomorrow, for tomorrow will be anxious for itself. Sufficient for the day is it's own trouble" (Matthew 6:34)
Worry, anxiety, fretting...these are all far too common words in our world today and in my world personally. But we don't just worry about today, we worry about tomorrow and the next day and the next year. For me personally, my worry is usually linked to finances. As the head of the household, I want to make sure everything is lined up so that my wife can stay home and raise our children instead of having to get a job and leave our children. So instead of just worrying about today, I worry about the rest of the week, month, year etc. I try to plan out my future to avoid anything happening to cause extra worry in the future when the bank account dwindles and we are out of options.
That being said, I don't think...let me rephrase that...I know that God doesn't want us to do this. First of all, he wants us to trust Him and rely on His strength instead of our own. But, I don't think He even wants us to rely on his strength to get us through the next week, month, year, etc...I think what he is saying is "My grace is sufficent" for today.
My worries are minor, I can't even imagine someone going through cancer or someone who has lost a child or a spouse. It has to be overwhelming to not only rely on God for the grace to get through today, but also rely on Him to provide enough to get through tomorrow and next week and next month but I think that is the key to what this verse is saying. Tomorrow will be anxious for itself or another translation says "Tomorrow will have it's own troubles" meaning that God's grace is new every morning (Lamentations 3:22-23). Let God grace and strength get you through today and then tomorrow, He will provide you new and unique grace to get through the troubles that He knows you will face for that day. God is sovereign and God works out everything for our good and His glory in His timing, trust Him and wake each morning asking Him for fresh grace for today.
