Don't worry. We all feel like that. Just remember that if you revise well and do as much as you can and ace your exam you don't have to retake it next time. All the worries will be gone and you'll achieve the best of your ability also even though all this revise is frustrating in the long run it will provide you with an excellent education and hopefully a great job.
Hope you are motivated now!
Good Luck with your exams (Oh and if you are religious try and pray as it helps me)
Revising can be a massive effort i know, but think of all the things you could do when you have revised. I have a treat timetable where i revise for say 9-12 in the morning and then at like 5-7pm. during the gap, i do something that i wamt, like seeing my friends or whatching a movie. This always helps me as if i don't revise i don't get to do the ting that i want:-) Also think of all the things that you can do in the future when you have all of your exams and when you've aced them and got the mark you wanted!
I don't if I'm honest, I just do what I feel like doing, I'm pretty sure I'll make it out okay anyhow because there's always another option, life will always take you somewhere. ...Sorry, that wasn't very motivational, was it?