It's never too late to establish boundaries. The question is whether or not the client will be willing to accept the changes.
It sounds like you do have a contract with the client which hopefully outlines your business policies and processes. I would probably put together an email letting the client know what you are and are not willing to do. Define or re-define your boundaries, what hours you will work and that you can no longer offer the service of going into her office more than once a month.
Let her know there are specific and scheduled times you can remote into her computer to complete work and that you writing to establish those specific times.
You can start the email by phrasing it that either because you have additional business or other client matters, it's now become necessary to schedule specific tasks for when they will be done.
And then you need to stick to those boundaries. If she request work be done around her schedule, then you politely let her know that you cannot do work at that time. I'd also remind her that you are a fellow business owner and not an on-call employee.
If you don't have time to take on additional work, then tell her that. Or let her know that she can contract additional hours the following month which will give you more time to adjust your schedule with other work to provide her with additional help.
The main thing is to remember that you are the boss of you. You are the business owner and it's perfectly within reason for you to tell the client "No" if it doesn't work for you. You don't "have to" visit the office, nor do you "have to" work around her schedule. You are choosing to do so. And if it's not working for you, then you need to choose to stop doing it.
There was a somewhat similar discussion on another thread recently that you might want to check out which I think has some good suggestions for how to handle boundaries with a client. I would especially look at page 2 of the discussion where Andrea has provided an email sample that you could tweak to send to your client. You can read it
HERE.