The Dark Lady has stolen the poet's heart and enslaved his friend as well, creating a triangular torment where the poet loses both self and beloved.
The friend is the poet's second self, his other identity. Her seduction of the friend is a second betrayal, a theft from within.
Abandoned and thwarted; the poet is undone from all sides simultaneously.
The sonnet uses legal and property language: the Dark Lady 'takes' the poet from himself, 'engrosses' his 'next self' (the friend). This makes the seduction a kind of theft and enslavement. The poet is 'forsaken' by both himself and his beloved. This inversion of agency—the poet becomes the object taken—reveals how desire unmakes the self. Love, in this formulation, is the loss of autonomous identity.
Line 8's 'torment thrice three-fold' suggests a mathematical impossibility of pain: three parties, three losses, compounded. The language of imprisonment ('prison my heart,' 'ward,' 'gaol') makes clear that the Dark Lady has become a tyrannical jailer. The poet's hope—that his suffering might bail out his friend—is touching but clearly desperate. He cannot trade his pain for his friend's freedom; both remain trapped.
Like discovering your best friend has been sleeping with your ex (or current) partner. The betrayal is doubled: losing the lover and the friend simultaneously. The poem captures the particular torment of romantic entanglement with a friend—the problem isn't just jealousy but the destruction of multiple bonds.