It's ok, mate, shit happens sometimes. You'll get over this and in a short time that presentation will be but a memory. I know how you feel, I've been there too. Once I had to do a short presentation and I completely blocked on my own name - I just couldn't say it! I resorted to use a diminutive in order to introduce myself to the group I was speaking to and carry on. The remainder of the presentation was an utter catastrophe anyway, filled with choppy speech and long pauses. Although I got better now at speaking to groups (I have to do that from time to time in my work), I sometimes still feel ashamed thinking about that presentation I gave. It, however, serves as a reminder of how far I've got now. So don't let this episode prevent you from speaking again if need be: I promise you'll do better!