I would really recommend that you go back at least one more time and talk with her about exactly what you've said here. It seems that the relationship up to this point has been good, but has been slowly going in the wrong direction in your view. Let her know that. Get her to specifically articulate WHY she sees getting a diagnosis is important. If she can't do that, I'm guessing that she's just frustrated and feels that you are spinning your wheels in therapy and she's grabbing at something or anything. Getting a formal evaluation is expensive and she should have a pretty good explanation/assessment of why you need to go through that process and how it will improve or enhance your treatment. Good luck with whatever you decide.

PS You might be thinking why go back if I don't agree, but I strongly urge you to go back and assert yourself in this situation. It's fine to leave, but give yourself the chance to stand up for what you think or believe. It's often a great opportunity to grow in therapy and if she doesn't seem willing to listen and explain adequately, you know for sure it's time to move on.

I think that one of the skills that is often helpful in therapy is to learn to say when it's not working for you. A straightforward statement at the beginning of therapy, as in "I'd like to talk today about X" may be all that's needed to correct the direction. You don't need to make a big deal out of how she's focusing on this an you want to focus on that, or be confrontational.