Aw, this made me sad - you were doing so well! I know it's after the fact now, but it sounds like you might have gotten down on yourself about missing the presentation, which made it harder to go to classes, and things just snowballed from there. That's partly what happened to me a long time ago. But some teachers can be understanding about stuff like this, and some universities even have policies for accommodating students with disabilities like social anxiety, which let you work around things like that.
Or there's always the option of just taking a zero on the presentation and working extra hard on everything else, and saying to yourself, dammit, I did the best I could!
Anyway, don't feel bad - I dropped out of college after 3 years due to increasing anxiety and depression, and after drugs, CBT, and 10 years of distance I was able to go back and finish my degree - all the credits I'd earned were still there. Things were much better the second time around.