i had sort of the same problem in yr12 and to be honest, a strict timetable didn't help me keep accountable at all, just made me feel guilty for not having gotten done what i had scheduled because i got side tracked with something else
obviously this is going to be very personal since it's ultimately a psychological issue, but the main thing to do is just diagnose what you need to get done (x from physics, y from chemistry, z from english etc) and just dedicate a whole study day to put your mind to just figuring that one thing out, whether it be familiarising with theories and mathematics for the light model in physics or getting a good intuition of acid base equilibria in chemistry
you'll receive other advice but i think it is generally best to avoid any type of pomodoro-esque scheduling (or anything that suggests like 1hr X, 1hr Y, 2hr Z etc) as it really detracts from building the intuition/understanding/grasp that you really need to excel easily