Stress
Oy, the stress. Money may not be able to buy happiness, but it's awfully hard to be happy with virtually none of it. The constant pressure to find some way to pay your bills and the nagging feeling of being stuck deep in debt can make you reevaluate how strongly you feel about acting.
Just do what your college professors advised you to do and channel all that anxiety into your work. Good thing the script calls for you to be nervous and awkward.
One of the biggest stresses of the career itself is the audition process. You'll never feel any less appreciated or wanted than you will in the audition room. It's not the fault of the casting director, as they actually want you to succeed (it makes them look good when you look good).
But in any given audition or callback (which is what you call the second, or even third, or sometimes twenty-fifth audition) they could have dozens or even hundreds of other people just as hungry and talented as you looking to book that two-line role on Arrow.
Then there's the waiting—not as in the restaurant serving you'll likely do at some point in your career, which has its own lovely stresses, but the actual sitting around. Believe it or not, there are seasons for auditions (no one hires around Christmas and New Year's, for instance), but even when you're "in season," there will be long periods of nothingness.
Then, when you finally do get called in, make it through the audition process, get in front of the producers, and book the role, it may still be months or years before the production is put on stage or screen. There's also the dreaded "on avail," where you agree not to schedule anything on a day that a production company might want to use you.
Our advice: fill that nothing with something. You're still an actor even if you're working a day job, so use your time productively. After all, it's only a waste of time if you aren't doing anything with it, and technically television is still considered "research."