I am sorry you are feeling such a crisis in your therapeutic relationship. That must be painfully difficult especially after 18 years and feeling as if you are regressing and that your therapy is becoming intolerable.
I am honing in on your statements of suddenly railing against the boundaries in the last few months, having been just fine previously. I wonder if this has something to do with your current diagnosis and/or your need to feel closer and more secure with your T in order to accept it. Often our need or desire to learn more about them is our longing to be closer and a search for equality and balance. To make sure that we are important and significant to them as they are to us. It is understandable that we desire for them to need us as much as we need them.
Your urge to resist and withhold could be a way for you to test if she is really there for you since you can't establish that by becoming more personally involved with her. Perhaps digging in your heals to see if she will step up to your expectations. But something has definitely shifted and it could be the slightest and least suspecting thing; even your T changing her hair color or clothing style could have an effect on our world.
I recommend instead of terminating to do some real self searching. If you journal go back to when you first noticed these feelings occurring and see if you find any hint as to anything particular going on at the time with you or your T. See if you noticed a subtle change such as did you sense her aging? Feeling or seeming ill? There are so many possible triggers but my T always asks me to pinpoint an emotion or two and to try to float back into my childhood and try to find the first time I remember feeling anything similar.
Your situation sounds very perplexing, but it sounds like your T is deiring to help you work through it.
Please don't think you are an idiot. If this were so easy to figure out we wouldn't need therapy in the first place. But whatever you have encountered deserves your attention. Is there a reason you feel you can't trust your T enough to just surrender to her again? Have you told her how you feel about her lack of self disclosure and your recent desire to know more about her? I think an 18 year relationship deserves that kind of exposure from you.
You say she "has become unreal to you" which suggest a deep shift. I wonder if thinking about what you have known to be true about your T, her good qualities, her warmth and caring you used to know, whatever once felt real that may rekindle what you feel is no longer true.
It probably sounds like I am taking many blind stabs at this, but I think the essense of your post suggests more self doubt that you are projecting on your T.
I hope this at least makes sense and is a little helpful in knowing that you came to the right place. If my words don't strike a chord, I am sure that someone elses' will. You can be sure of that.
Keep posting and expressing as much as you want.
JM