Not 'taking thought of the morrow' has been a key text in my life, though often I have failed it. But a question which surfaces every so often is: how do we plan, and organise for the future, whilst not worrying beyond the evil of this day? I have frequently said we must do both (not worry and plan), but then kind of held them in tension: trying to do both, and then trying to avoid the apparently unavoidable consequence of being concerned about the morrow: what will I need, what if it doesn't work, what if, what if...
I think the mistake may be
investing oneself in the planning for the future. That is to say, some planning is always needed - but then we tend to invest our hearts in it much more than we realise, as if our plans are the essential element in tomorrow and if they don't happen then it will disaster - and as a result, fretting then takes place to ensure that they happen. We start living in that hypothetical tomorrow - and a hypothetical tomorrow has so many more possible disasters than the real tomorrow will actually bring, so we start to deal with those unreal multiple disasters and find ourselves exhausted. Our thoughts of tomorrow become dominated by the urgency of tomorrow being the shape I planned for it, not with the thought that God is present there to provide.
I suspect the key is to plan for tomorrow, and next week and month and year - but 'on paper' only. Draw out the necessary plans, but invest our heart in God in today. We can plan for tomorrow and live in today. Once we have planned and moved whatever pieces into place as far as we can, we have to say to ourselves: "But tomorrow does not exist for me, it isn't real yet - and therefore stop worrying about it, instead invest your heart in what is real: today"