Maybe you can make up the missed presentation further along the semester, or else ask about extra credit assignments that you can do by yourself. Ultimately, I don't think this will make or break you as far as passing the class, provided you do try to redeem yourself and do your best from this point on. It would be a good idea to talk about this problem with the school though, either with the professor in question, or else the health department.
And yes, I used to miss a lot of classes (and a couple of presentations) at university. In fact, I think almost all of my final class grades were lowered by one whole letter due to my countless absences. I would get panicky simply walking in the door - it was as if I couldn't handle that all-eyes-on-me feeling. Often I'd show up to class late because I'd be in the bathroom mentally psyching myself up before going in, which of course makes the whole thing worse, since walking in late more or less guarantees that you're going to be a spectacle.
I can remember one semester when my anxiety really came to a head. I'd been attending classes all throughout it pretty regularly, but on finals/exam day it all got too much for me to handle so I didn't show up at all, which caused me to fail every one of them by default. Just another example of wasted time and ridiculous self-sabotage attributed to SA. Infuriating, when you think about it.