Should I quit?

leave 7cm for 360 for twitch-shooting pro's. You shouldn't go below 15 cm if you want to have any accuracy at all. That's at 1x zoom (outside of ADS). Always leave zoom-scaling to be relative (no multiplier). That is, at 4x zoom, it should take roughly 4x more to go 360. The idea is that going same distance on screen should always be the same distance on your mouse pad, otherwise you will have big trouble with consistency. You can double check it like this: outside of ads, move your mouse to a point you see at the edge of your screen, then go ADS (with some scope) and move your mouse to a point on the opposite side of the screen. Your mouse should move the same distance and end up where you started.

If you want to adjust faster, you may even want to make the sensitivity equal your desktop sensitivity - that is going half the screen in desktop should equal moving half a screen ingame (the test i gave earlier). But for that, you would best have a mouse with adjustable dpi, since that's the only way to change desktop sensitivity.

Tuning sensitivity and getting used to it so that you can snap right on target wherever it is on screen plays a very big part in getting consistent at aiming. Should take at least half a year with every day practice if you're new to precise aiming.

Also a BR may not be optimal for training, since you get very little shooting in this genre. Try a deathmatch shooter for training/warming up. Just make sure you make the sensitivity exactly the same across all games (equal distance for a 360 is a good start, but ideally you would want to have same "pixel travel per inch", that is turning to a point on edge of screen should be equal, because that's what's important for most of the aiming ). If you still want to play BR's then get into a habit of aiming at various objects constantly when you're running or doing nothing. You don't even need to shoot, just make sure you aim the objects accurately with as little adjustment as possible. The smaller the object, the better. Might as well add in some running around a rock and making sure your tracking is on point too.

P.S. When I started playing PubG, I started off with too high sens and was having trouble. Then I went to ~2 meters for 360 in ADS (so roughly 16 meters with an 8x scope 😉~  - a 360 with 8x is something you would never ever do anyway). That was perfect for everything, except very close combat (where ppl could just run around me and I couldnt follow, but I simply avoided CQC and that worked well enough). I increased it later when I got a bit more precise. 30-50 cm/360 for no-zoom ADS is about perfect for me. It's better to start too low than too high. Just make sure the mouse pad is at least half a meter wide.

If you play a lot of shooters later on, your wrist will thank you for learning arm-aiming. Wrist aiming busts your wrists sooner or later (at high sens, you aim just with the wrist and precise aiming usually includes 'locking' the wrist by flexing the muscles, which, if done too much will bust it). Arm-aiming may tire your arm muscles somewhat at start, but they're made for extended use.

Wrist-aiming may be a little faster (smaller mouse travel is inherently faster), but arm-aiming is simply more precise and, after some initial getting used to (most ppl are not used to using arm muscles at all for mousing), easier to learn. And considering that even most fps pro's use arm-aiming (20+ cm for 360, even in overwatch), it's probably the optimal way for aiming in shooters overall.