I’ve always had very bad test anxiety and after taking my AP tests, I knew I didn’t do as well as I could have because I was so anxious and felt like I was gonna either pass out or throw up.
I RECENTLY (after the school year ended) found out that I have GAD and that combined with my poor eating habits, lack of sleep, stress, etc. most likely caused my test scores to be lower.
I was planning on taking AP Calculus BC for senior year but after getting a 3 on the AB one I don’t know if I should. I was looking forward to it but I don’t know 
should I tell my teacher about my GAD or am I just making excuses??
I’m supposing you mean BC, and that’s an emphatic YES.
I slept 3 hours before each of my SAT’s; I feel really nervous before big tests too, and AP’s were no different.
But don’t worry! Many kids at my school get 2-3 on AB, then get 5 on BC the next year, and our school is below average. AB is a very harsh exam with about 25% 5’s, and many people do poorly on it. Conversely, BC is one of the most forgiving exams with over 48% of people getting 5’s. And there’s always the opportunity to bring up the AB score with the BC subscore, which over 50% of people get 5’s on. I believe the AB subscore on BC is easier than the AB exam itself, but colleges look at them the same.
I am confident you will do fine. With an AB foundation already in place, you are already ahead of many students. Since over 60% of the BC Exam is AB material, relearning AB in the first semester of a BC course will allow you to reinforce your knowledge, do well in the class, and ultimately succeed on the BC test.
Even though I was really nervous, I got a 5 on BC with subscore 5
How were your grades for Calc AB? That is more important than your AP score for college admissions.
Are you getting medical help for GAD? Do it now in HS if you can.