Workplace Problems

I was supposed to attend a meeting last week with HR and my manager. But I really wasn't in the right frame of mind. My GP referred me back to the psychiatrist I saw in February, and she wrote me a certificate saying I cannot attend.

End of story right?? Nope!

Because I have a Doctor's note saying I could not meet with HR, my workplace feels I am unfit to work. I am seriously regretting getting this note. HR still wants me to meet with my Manager as apparently my behaviour in the workplace needs to be discussed, but as HR is unable to, they want the 2nd head of the company to attend.
Slightly pissed about this, if people just told me when they have a problem with me it wouldn't have built up to this. I'd honestly rather deal with HR than the über boss, which would be even more terrifying for me.

And with all this, I can't get hold of my psychiatrist. I have no idea what to do and I'm freaking out.