On not assembling the whole thing
“Do not disturb yourself by picturing your life as a whole; do not assemble in your mind the many and varied troubles which have come to you in the past and will come again in the future.”
The instruction is oddly specific. Not do not worry, but do not assemble — he is describing a particular manoeuvre of the mind, which is gathering separate troubles into one pile and then attempting to carry the total.
No such total exists anywhere except in the assembling. The past ones are finished and the future ones are conjecture; only the present one has any weight. He wrote this to himself, at night, on campaign, which suggests it did not always work.