Is cutting the Depressor Septi muscle my only option for a horizontal crease above my upper lip? (photos)

Unfortunately, this is a question that is too difficult to answer with just photos for reference.  In general, cutting the depressor septi nasi would help with your concern if it truly is overactive.  See a board certified plastic surgeon.  Good luck and be safe. John T. Nguyen, MD, FACS, FICSDouble Board Certified Plastic Surgeon

The photographs are inadequate to determine if there is an active depressor septi ligament present between the tip of the nose and the upper lip. Releasing the depressor septi ligament is performed at the time of the rhinoplasty procedure. For many examples, please see the link and the video below

The crease may be created from a long caudal septum, have you checked with a rhinoplasty specialist? If this is the case reducing the caudal septum should reduce the crease.

Since you've developed it only few months ago, I recommend trying injections such as Botox prior committing to surgery.