Being so full of your love, I deliberately sought bitter experiences to prevent complacency, like a healthy person taking medicine to avoid future sickness—but this provoked disease instead.
The opening quatrain uses appetite and medicine as parallel metaphors. To make appetite keener, we add compounds; to prevent sickness, we purge. Both are strategies of excess designed to control fate. The speaker uses 'bitter sauces' as the compound that should have inoculated him against love's sweetness. Yet the paradox is surgical: the preventive medicine induces the disease it's meant to prevent. Love itself becomes the poison.
The couplet crystallizes the lesson: 'Drugs poison him that so fell sick of you.' Love is not a condition needing treatment; attempting to treat it through policy (deliberate strategy) only poisons the lover further. This reverses 117's logic: the transgressions weren't admirable tests but self-destructive errors. The speaker has learned that love requires acceptance, not management.
You're so happy with someone that you actually feel scared of the happiness—like you're waiting for it to collapse. So you start small fights, create tiny problems, to 'prepare' yourself for loss. But instead of preparing yourself, you just create the loss you feared.