Can't relax while singing...?

I'm having the worst time ever at the moment. High notes and singing in particular are not the major issue. My vocal coach tells me to relax, but the terms they give me were too vauge.

The bad thing about me is that I overthink and tense up, which cuts my performance quality in half. For example, this morning I was singing and rehearsing, I was just focusing on the words of the song, I just sang naturally. It was easy, nearly effortlessly, and NEVER painful. I even sounded pleasant and good.

Then this afternoon, I was overthinking. Always thinking "my diaphragm is not moving, breathe" and "You're flat or you're sharp" when I'm not. This also made singing harder than usual, and more effort was required. I was even singing the same song I did in the morning which was easy but now hard because I was overthinking!

What can I do to make myself relax while singing?