Every relationship with a mother seems to be different. So I can't really offer you any 'cookie-cutter' advice on how to deal with it. But, my mum sounds similar temperament to yours. I'm 29 now and she can still bring me to tears sometimes.
Only thing I have found that works is standing up to her. Not letting myself get buried under her comments. My mum constantly compares me to my brother who did slightly better than me at school... but he hasn't gone to uni (so ner!).
It's hard to find a happy medium with overbearing mums. But, please! Try not to let her get to you, it might result in you giving up because you feel like you can't please her. You might need to be honest with her, or write her a note. Explain that you are also upset about your results, sorry that they're not as good as she had hoped and that you will try harder.
I hope some of what I said helps.