After several cycles of yoyo dieting, my skin has become dull and a bit looser/saggier. Advice?

Microfractional laser treatments can benefit skin quality and resiliency through collagen stimulatory effects. In our office, we use combination platforms for Clear and Brilliant added to PRP, fractional RF, and filler placement in order to enhance the skin. We use Clear and Brilliant laser to improve sun spots, post-inflammatory hyperpigmentation, atrophic skin, and skin wrinkles; in addition to scars.
Best, Dr. KaramanoukianRealself100 Surgeon

morpheus, threads, ultherapy and therage are used on my younger clients to get tightening. facetite can be used to give you more contour. at home dermarolling should be used daily and proper skincarewith cosmelan and enlighten peels (see emeragecosmetics.com where we do free skincare consults daily). 
With significant neck sagging you would need a neck/face lift with muscle tightening (platysmaplasty). at the same time lasers (fraxel,  co2 or erbium( or microneedling RF (fractora, intensif) can be . performed to improve the crepe skin or wrinkles.  You need a formal consultation, as the neck can be treated with a combination approach:
1. micro needling and lasers for texture
2. botox, disport or xeomin to help platysmal bands
3. intensif or fractora RF microneedling for crepe skin and lifting
4. fillers like belotero, restylane silk, juvederm volbella to improve lines and folds
5. neck/face lifting if sagging is prominent or early starting +/- platysmaplasty (muscle tightening)
6. J plasma/Renuvion, thermiRF, inmode facetite for internal tightening
7. ultherapy and thermage for external tightening
8. regular Venus legacy, exilis ultra, or inmode forma treatments with facials to maintain tightness
9. PDO novathreads or silhouette instalift cone threads for lifting and long-term collagen production
see an expert for a full evaluation and treatment plan based on your individual situation and goals.
Best, Dr. Emer