Before student teaching began, I was undeniably stressed out. Even my face decided it wanted to break out more than usual. I was constantly upset, sad, frustrated, angry, and anxious, and I couldn't figure out why. Your subconscious is an interesting place, and sometimes it disagrees with what your mind is trying to tell it: "I'm okay, I'll be fine, this will be great, etc." Meanwhile, the subconscious is retaliating by yelling, "Freak out! Wait, no, calm down. No, panic! This is going to be awful! How will you get through this?!"
I knew that once I began my placement I would be fine and everything would work itself out (as it ceaselessly always does). I haven't begun teaching yet, I will this coming Tuesday by beginning my Holocaust unit centered around the nonfiction novel Night. I'm still stressing a bit about it, but I'm also crazily excited, almost overwhelmingly so.
So here's some advice, for myself included: Don't stress! This may sound cliche, but life is literally too short to worry about things you know will eventually be fine in the end. Learn techniques that uniquely calm yourself down, like maybe go for a run, read a book, watch a movie, hang out with a loved one--anything to keep your mind from consistently wandering back to the thing that's weighing down your mind. Also, something that helps me, is be organized. If you're fully prepared for something big about to occur in your life, it makes that thing a lot less intimidating.
Enjoy yourself! If you're going through a rough time just remember "this too shall pass." Life always gets better.