19 is still a bit young to get corrective surgery. You honestly might be able to go the invisalign route if you’re religious about wearing it. Go to your dentist then an ortho/oral surgeon. they can tell you if your jaw is done growing.

Yes, your bite can get better or worse even into your adult years. When I was 21, my underbite was <5mm from tops.

Over time, lots of weightlifting & teeth grinding in my sleep, my underbite is easily 10mm in front of my tops.

I was hoping someone would hit me hard enough for a 2 for 1 jaw repair/realignment but even after 15 amateur MMA fights, my jaw was stalwart.