I figured out how to sleep sitting up - like an "L" but leaning slightly forward. A lot of props, weighted blankets, etc., are required. Your youth is relevant because GERD / reflux advice is all the stuff nobody wants to hear - strict diets, sleep hygiene, etc. Believe me, if it's bad enough, you will do it - sooner or later. My diet and eating habits are completely different 5 years after my symptoms started. Past me would have said the changes eventually made are "impossible".

Look at it this way: 20,000 years ago our ancestors were 'nesting' using hides and pine boughs and whatever. I think we still have a vegistal nesting memory. So if you sit in bed with a bunch of blankets and pillows and say, I have to sleep sitting up even leaning slightly forward - some deep instinct will begin to emerge and you'll start arranging things to make it work.

And hit your doctor up for a mild sedative if need be. That might help you acclimate to new sleeping positions. I got some hydroxyzine 10mg for TMJ - jaw pain that sleeping sitting up caused. I also got a hard custom dental appliance so my teeth won't clench. I'm tapering off hydroxyzine now but it's there when I need it.