It was very hard for me. I was first diagnosed in 2017, and I rejected the idea. Then two different psychologists came to the same diagnosis. I wasn't ready to accept it. The last psychologist asked me why I rejected it, and my answer told him that I had no clue what being transgender meant.
I spent two more years in therapy with him as he explained what I misunderstood and shot down myths and disinformation. Then I asked him why he thought I was transgender, and he explained various behaviors since I was a child that I had told him about, and showed how those connected to his diagnosis.
Eventually, I saw what he saw, and what others had seen as well. Then therapy was about: now that I know this, what do I do about it? More therapy to discuss options, possible outcomes, obstacles, available services, and support.
Eventually, I decided that if this is who I am, then so be it. After that, I stopped questioning it. It became about what I could afford to do and what I wanted to get done.