How can I best perform this job?

I have been a legal secretary and loved it for many years. It is second nature, I have lots of free reign because I know what I am doing, and I don't have a lot of contact with the general public, not much answering of the phones, just handed a file with what to do generally, know exactly how to round up to the next task based on the one at hand and exceed in that area. For reasons that are irrelevant to my question, I have made a career change and am now Admin Asst in a very newly opened rehab/nursing center. I know very little about the workings of this type of business, and once I have learned more feel that I am capable of being efficient in that area as well. However, organization is lacking here, being a new business, and the boss seems to try to have control in all areas, down to how you staple return envelopes onto mail. I am used to being able to think on my own and have only been here a week but am having 2nd thoughts . . new career is exciting opportunity enticing but??

I am scattered continuously in many different directions, have no "normal" and consistent day, have no free reign at all, and have to relieve the switchboard (which I hate intensely).

Welcome to my world. LOL I'm an admin asst. also but I work for an HVAC contractor. The admin asst is typically the lowest ranking person in the office and therefore has no free reign at all. There's no such thing as a "normal" consistent day when you're an admin, well...it's that way for me anyway. The admin asst is expected to also be the receptionist and the secretary so most of your days will be chaotic. Every single day I have at work is utter chaos because I'm the secretary, the receptionist, and the office computer geek (read unofficial IT person). So...at any one time I'll be answering 3 calls at once, programming, reviewing city permits (another part of my job), and trying to drive away the employee standing at the front counter who wants me to jump up and fix his busted computer immediately. The reason I do programming is because my employer uses in-house created software for scheduling. It was created by my boss but he doesn't have time to do the upgrades anymore. Believe it or not...I didn't know the programming language when I was hired. Learned it on the job, still learning it in fact...2+ years later.

you've been in an ethical, highly responsible position and now have to implement the same for an owner/operator who shows signs of anxiety/fear of failure - so sorry. You do, however, know the 'process' of getting all points covered, documenting each of them and making sure all details are available to whomever needs to see/use them - implement what you know, documenting the process so that the O/M can, over time, release the need to double/triple check all things for peace of mind. I came out of that sort of thing myself, and this process, while generalistic (I don't know either of your work environments), did work up to the point that my manager felt I was showing her up.
On the other hand, you're new enough that moving from this job would not count against you and, by most decent employers, would be counted as a smart move on your part.