My own heart's temperament no longer agrees with me; it seems this matter has gone very far.

This couplet captures that quiet, startling moment when you realize you have lost control over your own emotions. The poet feels estranged from their own heart, as it refuses to heed reason or walk in step with the mind. Saying that the matter has 'gone far' is a poignant confession that love or sorrow has burrowed too deep to ever be pulled back.