Possible Clues:

• Lover is worn out with minimal energy
• Princess has aged no end
• Lover of 17-Across
• 'Tristan und --', a Richard Wagner opera
• Legendary lover admitted dealing in drugs
• Opera character whose best-known lyrics begin “Mild und leise, wie er lächelt” (Mildly and gently, how he smiles)
• Heroine, tragic victim of hubris? Old enough to see through this
• Princess is no spring chicken, last in line
• 'Tristan and ...' (Wagner opera)
• I betrayed eastern princess of legend