It isn't an overreaction at all. A Levels are very stressful as it is the first time in education that a student really has to self-motivate and learn independently.
So..
Firstly : don't panic!
Homework will take time to complete. A Levels are a FE qualification and are the stepping stone to Higher Education Institutions, if this is the path you choose to take, so it shouldn't be a walk in the park and homework will no longer take you half an hour to complete. You'll be expected to research and read around your subject to consolidate your subject knowledge.
If you're really stressed talk to your subject teachers or your mentor, and make yourself a revision timetable! Most importantly: make sure you take some time out for yourself!
It is crucial you have some time to yourself, play sport or relax doing something you enjoy and spend some time with friends.
This will reduce your stress levels, and if your time is managed correctly and you know when your deadlines are, things will seem more manageable.
If you manage to do this, you will get there.
If you map where your strengths and weaknesses are, you will be able to revise more effectively and increase your chances of success!