Here is the whole passage in context:
Matthew 6:25-34
25 “Therefore I tell you, do not be anxious about your life, what you will eat or what you will drink, nor about your body, what you will put on. Is not life more than food, and the body more than clothing?
26 Look at the birds of the air: they neither sow nor reap nor gather into barns, and yet your heavenly Father feeds them. Are you not of more value than they?
27 And which of you by being anxious can add a single hour to his span of life?
28 And why are you anxious about clothing? Consider the lilies of the field, how they grow: they neither toil nor spin,
29 yet I tell you, even Solomon in all his glory was not arrayed like one of these.
30 But if God so clothes the grass of the field, which today is alive and tomorrow is thrown into the oven, will he not much more clothe you, O you of little faith?
31 Therefore do not be anxious, saying, ‘What shall we eat?’ or ‘What shall we drink?’ or ‘What shall we wear?’
32 For the Gentiles seek after all these things, and your heavenly Father knows that you need them all.
33 But seek first the kingdom of God and his righteousness, and all these things will be added to you.
34 “Therefore do not be anxious about tomorrow, for tomorrow will be anxious for itself. Sufficient for the day is its own trouble.
What of the apparent instruction to worry about today?
I think verse 34 is encouraging us to remain useful and busy for the kingdom of God in the present instead of worry about the future. However, even if verse 34 could be said to instruct the believer to be anxious about today instead of tomorrow, verse 31 gives no such time requirements, but simply restricts the believer from being anxious.
Are we to not be anxious only about the essentials or does Phil 4:6; "Do not be anxious about anything" clarify this passage?
The listing of certain essentials (food, drink & clothing) aren't meant to show what things can or cannot be the objects of worry. I think Jesus refers to these essentials in order to restrict all worry regarding the things of this life. If worry isn't allowed for these important and life sustaining things, what would it be allowed for? It seems absurd to have Jesus restrict the believer's worry regarding these important items, but to allow worry regarding where I will sleep tonight or what someone thinks of me.
Even without Phillipians 4:6, I can't imagine worrying is allowed for anything, especially in light of verses 32 and 33.
When I'm racked with stress from what I'm going through at work, am I just plain sinning, or is this mostly just the normal human experience?
I guess that depends on what the source of the stress is. If the source of your stress is the concern that God doesn't know what you need and cannot provide it, then that would seem to be out of line with what Jesus is saying.
Those are my thoughts concerning the passage and your questions.
In Christ,
Dave