Yes, I have been feeling like this alot too. I have always got high grades in mocks and felt relatively happy with them and didn't really feel this lack of confidence before. I think it may be because you're in a slightly different frame of mind (you know it isn't simply against your peers and you won't be going through it in class etc.). Also, if you're a bit perfectionist like me, it does make it worse (I can make one tiny mistake and feel entirely unhappy about my exam performance). Just know that you're not alone and many other people, especially those who do well, feel this. I think, as someone told me a few days ago, its also important to realise that our final exams are rarely our best performance (i.e. the best piece of work you've ever done) and if it's not, that's normal and you can do perfectly well in it. We all feel a bit deflated anyway after a final exam because you realise all your work over two years has culminated into 1-2 hours, so this can add to it too.