Now I am very puzzled

The only thing I can advise is to do the best you can. You can't ask for more then your best, when you do not understand a subject. What you need to do in the future if you find a subject/class to be very difficult is seek some additional help, like tutoring. Your university counselor should be able to give you a list of free tutoring. Try and do this next time ahead of time so that there is room for the additional learning. Take your test and tell yourself you did the best you can do and if it means failure, well then you could not have done any better. See if it were because you never studied and did not care then you would have reason to be depressed. Do not be depressed when you are putting your best foot forward. That best foot may require a bit of help next time. Good luck to you and I wish you well.

First Take a deep breath and then let it out! Now do the subjects or the parts that you Know is the easiest or the fastest to do. and then work your way toward the harder projects. You can do this.

Just study, and read, keep reading it until you get it, and if you cannot understand what it is, ask someone about it. Don't become depressed over things because I have been there and it's not worth it, you are only hurting yourself by becoming depressed over nothing. You are risking your health and your well being if you become depressed over things that are beyond your control, or if you become depressed over things that you don't have to become depressed over.

have a plan.planning will make you manage your life easier~so do the first step-write a milestone or your schedule properly with the preparation of your exam along with the homework.its only takes e few minutes.manage your homework according to the difficulties, from easy to hard and there are many solutions provides-seek for an assistance-your friends, lecturers/teachers, online~calm down and do not let yourself drown in a depression flow. all the best!