When you’re angry or frustrated, are you beside yourself or besides yourself? Here’s the difference:
BESIDES means in addition to.
Besides me, only three people showed up at the meeting.
BESIDE means next to, alongside.
At the meeting, I sat beside a woman I had never met before.
However, the expression beside myself (with frustration, for example) strikes me as odd. Obviously, it’s idiomatic; you can’t physically get next to yourself, no matter how hard you try. But if you are sufficiently frustrated, you might feel as if you have been torn into two people. I’m just guessing here.