Here's part of what Edwards wrote in response (slightly edited for readability and with emphasis added). His advice to her is good advice for us all.
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June 3, 1741
Remember that pride is the worst viper that is in the heart, the greatest disturber of the soul’s peace and sweet communion with Christ; it was the first sin that ever was, and lies lowest in the foundation of Satan’s whole building, and is the most difficultly rooted out, and is the most hidden, secret and deceitful of all lusts, and often creeps in, insensibly, into the midst of religion and sometimes under the disguise of humility. [Therefore,] always look upon [the truths and grace] that have these two effects: (1) those that make you least, lowest, and most like a little child; and (2) those that do most engage and fix your heart in a full and firm disposition to deny yourself for God, and to spend and be spent for him.
If at any time you fall into doubts about the state of your soul under darkness and dull frames of mind, ’tis proper to look over past experiences, but yet don’t consume too much of your time and strength in poring and puzzling thoughts about old experiences, that in dull frames appear dim and are very much out of sight: but rather apply yourself with all your might, to do an earnest pursuit after renewed experience of grace, new light, and new, lively acts of faith and love.
One new discovery of the glory of Christ’s face, and the fountain of his sweet grace and love will do more towards scattering clouds of darkness and doubting in one minute, than examining old experiences...for a whole year.
When the exercise of grace is at a low ebb, and corruption prevails, and by that means fear prevails, don’t desire to have fear cast out any other way, than by the reviving and prevailing of love, ... for when love is asleep, the saints need fear to restrain them from sin. ... But when love is in lively exercise, persons don’t need fear, and the prevailing of love in the heart naturally tends to cast out fear, as darkness in a room vanishes away as you let more and more of the perfect beams of the sun into it (1 John 4:18).
—Source: Jonathan Edwards, “To Deborah Hatheway,” in Letters and Personal Writings, ed. George S. Claghorn, vol. 16 of The Works of Jonathan Edwards (New Haven, CT: Yale University Press, 1998), 91-95.