What I think are malar festoons appeared very suddenly over my left cheekbone. Could I fix it myself? (Photo)

This subtle hollowness is due to lack of volume and is very common. The technical term for this is Orbito-zygomatic groove and even young people have this anatomic feature. Fillers are ideal in this area, Restylane or Radiesse work very well with minimal swelling. 

This is not a festoon, this is a crease under your eye which many younger people have and can be influenced by age or weight changes.  Also you have dark circles that can be improved.  Fillers and peels will help most for this. you will never "cure" this you will always have but will be improved with noninvasive treatments.  An at home peel, and at home roller protocol will help to improve color and tighten this area.
There are many things you can do to treat under eye hollows and dark circles. A surgical option to remove skin and/or fat will be the best long term.  There are many things you can do to treat under eye hollows and dark circles. A surgical option to remove skin and/or fat will be the best long term.  However, non invasive less downtime options such as fillers, lasers, microneedling/PRP, cosmelan peels and stem cells can all help give some improvement. I suggest. You get a formal consultation to see what is best for you and your goals.  Best, Dr. Emer.