I had an assessment centre for a very competitive graduate scheme (in London in finance). Back to back interviews/presentations from 9-4pm with only 15 minutes for lunch because of my extra time. My stammer was severe that day and it was exhausting. I was blocking hard on virtually every word I said. I cried afterwards, not because of the emotional toll on me, but just because it was physically and mentally so difficult for me. But I got the job!!! Keep trying and believe in yourself :D There are people out there who will be understanding of your 'quirk' and will respect your perseverance.