I was in an identical position 3 weeks ago and I shared some of your concerns as well. I can tell you that it seems scarier than it really is once you're in it. You shouldn't be worried about getting 'stuck', I did a number of times and quite often would get hints and directions from the interviewer. Never give up, even if what you know you don't know the answer right away, let the interviewer see that you're thinking and try to engage a discussion.

And really just try to take it easy. I was also under a bit of pressure, but think of it like an experience, even if you end up not passing. It's just another company and the feedback of 5 random people.