Your boss sounds like a concerned adult, with a few years on her shoulder, who's concerned about your mental health. Discussing your anxiety issues with you shows a concerned supervisor who wants to better her employees. If she wanted to (excuse my impoliteness here) can you, there would be no personal meeting. Believe me, I've worked in corporate America for the past fifteen years, and I know the feeling of being a number, rather than an employee. Due to the *at will* policy in Human Resources, you could be dropped at any moment, however employers vary on how they manage their employee teams.
Since you're young, at just 22, (and this is a nice accomplishment to get a good job at that young of an age -- grin), I'm fairly convinced your supervisor is a mother, with children of her own, and feels maternal to you. Likewise since she's a nurse, she feels compassion to her patients and employees in general, so although you may feel anxiety that the world is going to end, don't think everyone at work thinks and feels exactly like you do. You might be feeling anxiety, however your boss might be feeling concern, about how to better her employees.
Although it may be hard, at the moment, to see your anxiety from another point of view, just know it's not as bad as you think.
Your supervisor was wise to send you to a counselor, who in turn, sent you to a medical doctor who prescribed you Lexapro at 20mg. Welcome to the world of SSRIs. Like you, I'm a professional in the workplace, with a college degree, and my job at times can be stressful. Like other realistic anxiety sufferers, I know my body's limits to handle stress on its own, and although it would be nice to feel I could handle anxiety without medication, I've accepted I'm better with the medication than without.
It may take up to 12 weeks for the SSRI to fully take into affect, however, you'll probably feel benefits in the next two to three weeks. If you experience side effects, discuss them with your doctor, however the initial weeks may have additional anxiety, but that heightened anxiety drops down after a few weeks.
You may want to view this time period as a way to make you a better nurse. Part of being a good nurse is to understand the role of the patient, and since you're the patient in regards to your anxiety disorder, you'll have this experience to focus on, as you work with other patients in the future.
Just know, things aren't as bad as they always seem (grin).