I'm concerned about my level of preparation for graduate school. I have decent grades (3.64 GPA, 4.0 in math), great recommendations lined up, and some OK research experience (no publications), but I really don't feel like I'm ready for the next step.
I'm interested in applied math, and was a physics/computer science double major for most of undergrad, switching into math just last semester. I'm taking Analysis 1 now as a first semester senior, and it is the only pure math class I've ever taken (though I've taken a bunch of applied courses and semi-pure courses, like advanced udergrad ODEs). I'm doing very well in the course, and I'm really enjoying it, but I feel that, because I'm so late in joining mathematics, I'll be really behind everyone when I get to graduate school, if I do get in somewhere decent. My scores on practice tests for the math GRE are in the mid 600's, and that makes me feel like a phony as well (although I haven't really studied that hard yet, maybe they'll go up? Also, its a good thing many applied programs don't care about the subject test...). Sure, I have some skills from my background that other students don't have, such as non-trivial software development and an ability to solve difficult physics problems, but when I look at the profiles of the 'good' people on this website, or talk to my friends that have been doing math olympiads since they were 3, I feel like there is just no way I can be good enough for a career in mathematics given my 'late' start.
Is it possible to catch up with others at this stage? Will I be doomed to a life of constantly being behind those who are smarter than me? Has anyone ever built a strong career from my kind of situation before?
I just wish I had some more time...
Impostor syndrome is not uncommon among people about transition to the "next step," or among people entering into or about to enter into new experiences that they perceive to be a result of skill.
Believe it or not, although graduate school is designed to be a "down-selection" of undergraduates, first and second year graduate students can be as clueless as anyone else. Don't believe me? Survey your professors.
Your abilities have come from a combination of skill, hard work, and desire. A major-specific 4.0 is not bad. Just because you haven't established a coursework background in certain subjects doesn't mean you cannot do so. Think about it like this: no matter what you do, pretty soon you're going to have to learn material outside the structured framework of a formal course of study.
Finally, practice GRE scores in the Math subject test are a poor discriminator. The test is so variable that scores can only be compared to other scores from the same test. A 600 on one test might be in the 95th percentile, while a 600 on another might only be the 75th.
Ultimately, regardless of what you do, you're going to have to trust your own abilities, and also trust in others' assessments of your skills. It's a very difficult thing to do, sometimes.