Why is half of my face and jaw looking so weak and causing one side of my face to sag? What do I do to reverse it? (Photo)

Hi, I’m 19 and one side of my jaw looks completely different from the other; it’s longer, weaker and affects my face, with sagging skin that is even causing one half to develop fine lines and droop down. It’s slightly awkward to close my mouth and my ear pops on that side when I open my jaw too wide. I have pain sometimes, but although I’ve always noticed this about my face it seems to be getting worse.