Its pretty common to have anxiety before going into something like this.
The best advice I got was from our district CO during inprocessing who said that the quickest way to get dropped from OCS is to start "self-evaluating" yourself. When you start comparing yourself to other Candidates in your platoon and getting the mindset that, because they are better, you are clearly not good enough you will start to fail. That kind of attitude is like Cancer and will eat away at your confidence, your physical and mental courage, and pretty much everything else that you will need to survive.
Do your best, always give 100%, act like you want to be there, and you will do just fine. I have never seen a case where the staff wasnt willing to work with a Candidate who may have struggled with something but clearly showed that they were giving it all and committed to improving themselves.
You have to realize that you will not be the best Candidate in the company and you will probably fail at something, its how your react to that failure is what matters. And when you go through a low point while there and you arent feeling very motivated, remember this: the person who graduates last in his class at OCS is still called a 2nd Lt.