Diagnosis as Validation?

I have been in and out of therapy for most of my life for a variety of issues that I could never seem to put a finger on.  I stumbled across this site and Melissa's book as my relationship to my fiance continues to decline.  I resonated with so much of what I have read thus far and am convinced that I have ADHD and it is negatively affecting my relationship.  I plan to discuss this with my therapist in our next meeting, but when I brought this up to my fiance, she was skeptical that all it provides is validation and nothing else.  In part, I feel a diagnosis gives me something to rally my efforts to improve around and find additional support resources (e.g., books, support community, medication, etc.) to help me in my treatment journey.  Additionally, these resources will help me keep a pulse on "reality" so I avoid getting so defensive and I can better listen and communicate with her as well as understand a situation that I am confused about.

Has anyone else encountered this resistance to a diagnosis?  How has your diagnosis helped beyond just validation?  Right now, my fiance is skeptical that a diagnosis will help anything.