So im gonna be honest with you right now. A levels are hard.
I did GCSE Bio, Chem, Physics and Maths and i got a straight 7's in science and 6 in maths. Im currently in year 12 and i do a level bio chem and psychology. and because i didnt get 8's/9's (A's or A*s) the courses were harder. Chemistry specificaly which is required for medicine is extremely hard without the neccessary backrgound knowledge. I really wanted to do medicine but now in year 12 my passion for it died and i am thinking of dropping chemistry.
My advice: if you want medicine bad, revise now, actually understand the stuff and create your background knowledge so its really strong and the courses would be a breeze for you. By the sounds of it you have the drive and the passion for it. Then A-Levels for you wont be as hard because you created the background knowledge. In regards to uni, every uni course is difficult but as long as you believe in what you are studying it will be worth it. there are many people in my school and i unis who are doing very well in the courses, it just wasnt for me.
Sorry if this wasnt helpful