I haven't been through the viva process yet, but I've been in to speak to my supervisors about how I should prepare. I'm not sure where you're studying, but I'm in the UK and one thing that my supervisors told me is that the viva is not a memory test. you can take your dissertation in with you and make some notes. In fact this is an important thing to do. But the examiners don't expect you to remember everything by heart.
It sounds a little bit like you're panicking. I know it's hard, but try not to. Take a deep breath and remember: You know your dissertation better than anyone else because you wrote it!
Good luck!