Sorry, this is going to be long.
I'm a U.S. undergrad in senior year, majoring in physics but with significant interest and experience in another (totally unrelated) field. For the last 2ish years, I had thought I would go to graduate school in physics and make physics my career but work in my other field of interest for a year or two first. I planned to apply this year just to see how it went (and if I got a cool acceptance maybe I could defer for a year). I'm no superstar - no prestigious awards or publications - but I have a lot of research experience and a >3.8 GPA (well, "had" on the GPA; see below) so hoping to go to a competitive school seemed not crazy, and other students from my school have taken gap years without negative effects.
Then this fall happened. Not that physics was ever easy, but it felt like I just couldn't do physics anymore. Not that I didn't want to, I cared so much. Just everything was beyond difficult all of a sudden. It felt like my abilities crashed to near zero and I don't even know why. I'd spend 30+ hours on problem sets and not be able to finish. I'd hand in everything late, realize I was super behind and skip doing a couple of assignments so that I could get back on track, and then just fall behind again. I almost got kicked out of senior thesis because I was making so little progress. It was miserable, I was miserable, everything sucked. I took the PGRE and my scores weren't rock-bottom but they weren't good enough for the schools I'd imagined going to. When it came time to actually fill out grad school applications I just couldn't do it. I really did spend a lot of time trying to write all the necessary materials. I just couldn't find any words to convince anyone why they would possibly want me as a grad student. I only submitted a couple of applications in the end.
That in itself is not so important since I'd planned to work before graduate school anyway, but before this fall I would have just applied for jobs in my non-physics field of interest. Now I'm wondering if I should forget that and only apply for physics-related jobs instead, so that I have some extra experience/proof that I know what I'm doing when I next apply to graduate school. It would also be nice to have my senior thesis not be my most recent research experience, because even though I think my advisor wants to help me I can't imagine he has much good to say about me after this year and I'm scared to ask him for recommendation letters in the future.
Will having worked in research during a gap year/years make a substantial difference on applications? Will a semester of bad grades (in my senior year, no less) forever lock me out of places like Princeton, Stanford, Columbia, Michigan, Wisconsin-Madison? I know there are lots of good graduate programs outside of the top of the rankings, but pre-last fall I'd nurtured dreams of possibly going into academia and I know name recognition really does matter on that path.
Also, has anyone else ever had a sudden crash like this? I truly don't know why it happened and I'm terrified that it won't abate this semester or could happen again in the future. Is this burnout? Is this random nonsense? Am I as stupid as I feel right now? I thought long and hard about refocusing entirely on my other field, but as much as I love it, if I'm honest with myself it's physics I really want in the long term. Part of my soul would die if I never saw another integral.
This sad, lost pigeon of a physics student would greatly appreciate any morsels of advice you could throw in this direction.