1. Diana is in her element when she’s in the classroom. She’s a natural teacher.
2. I love being in the country. When I’m in the city, I’m lost. I’m out of my element.

Compare to: out of (one’s) depth; fish out of water; in (way) over one’s head. Out of one’s element describes being uncomfortable in an environment, whereas out of one’s depth describes being asked to perform beyond one’s capability.