Started new job. My boss thinks I look stressed and am too enthusiastic? Advice!?

You're doing the right thing, just try to be calm and normal and don't be too loud. If, after a couple weeks you are still having anxiety and they are still telling you you are too enthusiastic, then honestly they have a problem which isn't yours and they are causing bad chemistry for you. In that case you might have to move on. When I tried working at JC Penney the trainer accused me of 'just standing there like a mute statue' but what did she expect when she was saying things like 'Are you ok?' 'What's wrong with you?' etc. I dumped that job fast, and this problem has never repeated itself. That's because, it was her problem and not mine. So I think it's your bosses problem and not yours.