I did the first (and hardest) part of my thesis proposal defense yesterday; a 1 hour oral presentation to 2 out of the 3 profs on my committee. That was followed by 3 hours worth of lab meetings - I guess that's what I get for joining two labs, not just one. After that, I met with one of my advisors. And after that many hours straight of science.... I'm still really really excited about doing my projects.
I know, I know, I still have about 4 more years of grad school during which to completely wear myself out on the subject. But it's so gratifying, after a month of stressing out and reading and working hard, I still want to be here, and I still want to do science. This whole thing has been a validating experience, which I did not anticipate.