Answer:
"loss with which heaven has been pleased to afflict you"
Step-by-step explanation:
The full sentence in the letter is
"Consider the good lady who has taken you under her roof. . . as your mother, as the only person to whom, since the loss with which heaven has been pleased to afflict you, you can now look up."
Thomas Jefferson, had entrusted the care of his three children (including Patsy) to Thomas Hopkinson, the mother of a friend, before he left for his official duties. He wrote a letter on November 28th, 1783 to eleven-year-old Patsy who was his oldest daughter.
In literature this technique of using mild or indirect reference to some harsh or unpleasant things/event is known as euphemism.