Go and see your GP.
You sound as though you are being unduly pessimistic. I cnat tell whether you are overdoing it or being realistic. I wouldnt sit exams I knew I was going to get U's for, but its common for people to panic and be pessimistic, then perform quite well, its just a way of delsing with nerves.
You cna still do a lot in the time you have left if you put the work in. Try your best and the rest cna deal with itself after, if you choose to sit the exams.