Hey! So proud of you for pushing along and getting the ball rolling for a diagnosis, a diagnosis can be really helpful for getting tools to help in college.
As someone who has autism, which isn't the same as adhd but still scary to tell someone, I had to tell my mom gently. When I told my mom I have autism I made sure to soften the blow as much as possible and leave room for questions.
That being said, here is what I recommend to vaugley tell your mom "Mom, I need to talk to you about something serious. It is something that is very important to me and something that I think will benefit me in life. It is something that I can be treated for professionally and that will benefit me if treated. I have ADHD, I have the symptoms of [fill in the blank], and [therapist] says that I may have it. We ran a test and it is looking like I do have ADHD. I would like to have an official diagnosis, because then I can go on medication which can help with [symptoms] and benefit me in college by getting tools that I need to succeed."