I am a guy with really long hair ever since I was your age, and I also took Accutane at your age, too (thirty-many years ago!). I've found that the most part of my hair loss has always come from brushing and detangling, obvious mechanical things, rather than nebulous things like diet, a drug side effect, shampooing too much or too little, or using cheap product. My hair still looks quite like yours. Maybe a bit more gray.

You don't have to worry about male pattern baldness at age 15. The earliest something like that might possibly kick in is around 20, and that's pretty early. So put that out of your head. That would just be silly to worry about, even if your Dad or uncle is bald.

Everybody sheds a lot of hair. When your hair gets long, that regular shedding can look extreme because there's so much more of it and it's coming out mostly all at once. With short hair, it's not a big deal because it doesn't get stuck in the brush and it just goes down the drain. But now, expect to get a handful everytime you shower.

My long hair has been great for over thirty years, but I still lose so much in the shower that it kind of freaks me out. And yet the rest keeps growing.

Some ways to minimize this kind of natural shedding are...

• Wear a loose ponytail at night (if you tie it up) to avoid pulling it.
• Use a conditioner with a serious detangler, which usually means favoring silicone/dimethicone. A lot of people here dislike silicones for some reason or another (maybe there's something in the sidebar), but it really helps protect aginst tangles and knots like nothing else.
• Try detangling in the shower when you do the conditioner--take a wide comb, pick, or the end of a "tined" brush (not like a horsehair one) and carefully separate the strands of hair by starting at the ends and working up through the conditioner-soaked hair. Once you work your way up through a section, you should be able to gently bring the comb (a wide one, not the Ace kind) down through the same hair with minimal shedding.
• Never pull through a tangle with a comb or brush. If your comb or brush starts to get caught on some hair, stop, identify the tangle, and undo it. If it's a knot, isolate it and detangle it from the other strands so you can cut it off just above the knot (on that single hair).

Anyway, I don't think you're going bald. I think instead your hair is just getting really long!