Nurse: My long life has taught me many lessons: mortals should not mix the cup of their affection to one another too strong,  and it should not sink to the very marrow of the soul, but the affection that binds their hearts should be easy to loosen, easy either to thrust from them or to bind tightly.
In the simple imagery of the mixing bowl and the knot, the nurse offers a vivid emblem of Sophrosyne, in the received sense of nothing in excess. Our loves must not sink to the marrow of the soul -- μυελὸν ψυχῆς -- but should rather be relaxed, like the reins on a horse, able to be pushed away, or tightly drawn in. Images invoking the polarity of looseness / tightness -- of rope, reins, love -- are woven through the play.
To love wisely, for the nurse, is to be in balance: able to love yet to leave. The sense of an affection and an affability that reserves to itself the freedom to be more, or less, as the lover wishes. Love, here, is a mixed wine, neither too potent nor too weak. Interestingly the word she uses for "love" in line 257, στέργηθρα, carries both the sense of "fondness" as of the love of parents for children, and "love charm," suggesting an efficacious influence or power to which one might succumb.
Thus, instead of easily mixing these elements, the nurse's language holds in suspense two kinds of eros: a love that obeys one's desire, and a desire that one must obey.
The nurse continues in the same vein:
Men say that a way of life too unswerving leads more to a fall than to satisfaction and is more hurtful to health. That is why I have much less praise for excess  than for moderation. The wise will bear me out.
"Too unswerving" is a fine translation of ἀτρεκεῖς, which can also be translated as "strict, precise, exact." Someone who is too exacting, too stiffly precise, is setting themselves up for a fall. The word for "fall" is σφάλλειν, a word Aphrodite also uses in her opening speech:
I honor those who reverence my power, but I lay low (literally: I trip up) those who think proud thoughts against me.Those whose wills can't bend will break, as it seems Aphrodite is unswerving in her demand for reverence. We might be advised to treat love lightly, but Love is too exacting to let us walk away. The ideal of self-control breaks upon the quandary of how to find a "moderate course" between Aphrodite and her antithesis, Artemis.
At the beginning and at the end of the Hippolytus, Euripides presents the two poles of love in the form of symmetrical deae ex machinae. Each is absolute, each is the negation of the other. Mortals negotiate the electrically charged space between them. It is in this atmosphere that the various models of Sophrosyne must be taken up, weighed, and examined for viability. The virtue that seemed blandly easy for us to practice in Aristotle's prosaic Ethics is ratcheted up very high in the tragic poetry enacted on Euripides' stage.