What procedure or treatment would be most effective for addressing my sagging cheeks? (Photo)

Thank you for your enquiry and sharing your photographs.
My impression is that the you do not need to blame yourself for not wearing the sunblock. This is not to underestimate the importance of sun protection, I am a true believer in its value and recommend it wholeheartedly to everyone who cares to listen!
I get the impression that the underlying problem is predominantly the moderate crossbite and the fact that the chin is sitting slightly more towards the left. This creates a more concave appearance of the left side of the face and a more convex appearance on the right. This may have become more noticeable due to facial volume loss and slight reduction in the elasticity of the skin.
There is slight laxity and fullness along the jawline which could be improved with liposuction.
I would also suggest discussing with a maxillofacial surgeon whether a centralising genioplasty would be a reasonable intervention in your case.
Volume augmentation using injectable fillers or fat transfer can be used but should be in my opinion moderate to to create the best results.
Minimal access facelift surgery is the type of intervention that lifts the facial soft tissue volumes through an access in the sideburn and can be combined with fat transfer to produce a different balance of the facial features.

Thank you for your question! You might be a candidate for a non-invasive procedure called Thermage that can tighten skin and stimulate your body to make healthier collagen—the building block that provides structure to your skin. The procedure uses radiofrequency technology called ThermaCool® that has been clinically proven to tighten and lift skin. Fast and easy, Thermage requires no downtime from normal activities. A combination of Thermage and non-ablative lasers can be a powerful nonsurgical treatment to help tighten and smooth skin all over the body. My office also has an innovative skin treatment which uses a combination of microdermabrasion and various laser treatments to help improve skin quality and appearance. I have also created skincare products that scientifically help improve quality of your skin. I suggest making an appointment with a board-certified plastic surgeon to discuss your options in more detail!