I'm thinking that perhaps when she was in her twenties she found herself subject to temptation she hadn't faced before. She decided as a teenager to follow certain principles, and they seemed very straightforward and sensible at the time, but years later when she had both the opportunity and the desire to abandon those principles, it all seemed much murkier. She needed to return to the clear-headed un-tempted state of mind that was captured in her letter to herself.
It reminds me of a major idea in Ignatian spirituality, that you cannot discern God's true will when at the same time you're desperately trying to make it match your own.
Or, as Jane Eyre has it:
"I will hold to the principles received by me when I was sane, and not mad--as I am now. Laws and principles are not for the times when there is no temptation: they are for such moments as this, when body and soul rise in mutiny against their rigour; stringent are they; inviolate they shall be. If at my individual convenience I might break them, what would be their worth? They have a worth--so I have always believed; and if I cannot believe it now, it is because I am insane--quite insane: with my veins running fire, and my heart is beating faster than I can count its throbs. Preconceived opinions, foregone determinations, are all I have at this hour to stand by: there I plant my foot."