The more you care about your interview, the less likely you are to pass it.
Interviews are all about maintaining a nimble, flexible state of mind, so you can react gracefully when hit with a problem you haven't prepared for. This will almost always happen on interviews with tougher companies.
If you view your interview as a "make or break" opportunity (which it admittedly very well may be) and go into it with a lot of anxiety, you will not have that healthy state of mind. When your interviewer goes off-script, your mental state will likely shatter.
View interviews as a chance for you to go in and learn from some very smart people, and if you happen to get a job offer, that's a nice bonus. This much more relaxed and positive mentality will reduce your anxiety.