Start preparing a discipline schedule
I know it sounds like shit but if you feel that you've gotten more lazy, then discipline is the best medicine, helps you study even when you don't have motivation.
Willpower is a muscle which needs to be exercised, look up biohacking, start employing multiple methods of studying and disciple maintenence.
Bad grades are common if you get into a high performing school, don't stress, stress impedes work, it's useless, try to enjoy yourself and be positive, so look after your mental health, If you neglect it then your marks will neglect you.
Be there for yourself, love yourself, encourage yourself. Enjoy studying, we are good at what we love.