In terms of improving concentration and time management, I find that the pomodoro technique is really helpful. You basically set a timer for 25 mins, have a 5 min break and do this 3 more times (so you've done this process 4 times in total) and then have a longer break, like an hour or something. It gets you being productive but it's not too intensive/boring/time wasting!
For not feeling overwhelmed, breaking big tasks into bitesize chunks is really useful. Projects such as coursework are just a lot of small tasks clumped together!
Also it's a bit cliche but doing past papers/exam practice and studying mark schemes is so key, I have friends who have basically told me this is can be more important than the content itself. Obviously, you do need to know subject content but exam technique can go a very long way.
Also speaking to teachers/friends/family etc can really help, you might have already done this but I knew it helped me reduce my stress levels personally and get good advice about things like revision/balancing work and life etc.
If you don't mind me asking, what subjects are you taking?
Hope this helped!