I was supposed to take an exam today at 10:15 a.m. (aka now) and even tough I was reasonably well prepared and have the option to repeat the exam if I fail it, I just couldn't get myself to calm down again to get ready and just go there.
I could have probably ask my professor if I could simply just take the second exam instead, or tell him that I overslept, but I just feel horrible because I already missed last week's weekly assignment class (includes mandatory presentation of the assignment once per semester) due to my anxiety and he offered me an extra time sloth to present mine, since I never got around doing that. He was really welcoming and friendly about it (I just briefly mentioned how it's been a hard time for me mentally), but I just feel like I've failed him. I'm also worried about him feeling played by me or that he thinks I don't take him seriously.

I know I should write him an email and explain my situation, but I just genuinely don't know what to say because I feel so horrible about not showing up, even after he was so helpful and accommodating.

I'm always anxious about failing my exams and performing badly, but recently, everything has just been so much worse. I'm also trying to get professional help and a diagnosis, but every psychologist and psychiatrist I've contacted so far is completely booked out for the foreseeable future.

I just don't know how to deal with this situation or what I should tell him.