How do I fix this genioplasty? (photos)

Unfortunately a revision might be the best option for you. Aside from the soft tissue issue, the chin itself looks uneven and tilted down more on one side. I would suggest another genioplasty, that will place your chin in the correct position, and possibly add some contouring to better shape your chin and jawline. Adjusting the placement of the chin will also help to tighten the skin and create more definition and less of the skin laxity.

Your mandible was severely undersized and it wasn’t just the front part of the mandible that was undersized the entire body was small. Without doing a job advancement simply doing a sliding genioplasty is going to have its inherent limitations.
Your situation was not an easy one to fix.
Follow up with your provider or have a few in person Second opinion consultations with other oral surgeons in your community.
Doctors go to medical school not magic school.
Best,
Mats Hagstrom MD

The images suggest that the vertical height of the chin could be improved as well as the 'dips' just behind the chin along the jawline which make the chin look the way you describe. However the 'feeling' of tightness etc is not something that can be objectively reviewed. Talk to the surgeon about contour improvement