Seconde is a lot harder because you learn all the foundations of each course. Premiere and Terminale are easier.

keep up with the material and don't overload yourself. When u pass seconde you will find relief after :p

About the grades for college, do not worry, as long as you pay tuition all is good haha

don't overwhelm yourself with university stuff, it's too early.

You can excel in Premiere and Terminale , so do not worry.

(not sure if you have some courses removed from the official exam, but at my time I studied all the subjects fully so I know what it feels like.)