I had a Skype interview...

I had a Skype interview... So on Tuesday I had a Skype interview for an internship position with my school. I spent all of Monday preparing what I was going to say for each question and felt excited/nervous on the day. The call comes in, and I immediately blank. I forget everything I've practiced and its pretty obvious I looked 'unprepared". Blocks, facial ticks, the whole 9. I had told my interviewer before hand, and she understood, however I failed to get the position. Anyone been in a similar situation, how do you guys prepare for stressful situations like an interview or presentation as I have several coming up. Thanks guys