I'm an undergraduate in my junior year, and I've been having an especially bad month when it comes to my mental health and academic performance. Two weeks ago, I discovered that I had two midterms, one major project, and an 6-8 page essay due on the same day. I explained my workload to the professor who assigned the essay, and he was kind enough to give me an indefinite extension on the essay (as in, I could "turn it in whenever it was ready", although I assumed that 1-2 weeks was what he would prefer). Since I was granted that extension, I've completely flunked the major project and received C's on the two midterms, all of which I prepared extensively for once I didn't have to worry about the essay. The professor has started hinting that he'd like essays he gave extensions on (he gave extensions to about half the class, since a good 2/3 of our university got the flu around then) to be turned in soon, since he's expecting to receive more essays from another class he's teaching, and doesn't want to be overwhelmed by grading.
It's been two weeks since he gave me the extension. I've spent nearly all of those two weeks attempting to write the essay and then completely breaking down. I start thinking about how kind he was to give me an extension in the first place, and how, by not turning it in by now I'm taking advantage of his kindness, and how my reputation among my department's professors will be ruined. I think about how this essay has to be perfect, since I've completely failed my assignments for every other class, and I have to get this one thing right. I wrote 4 pages of the essay and then deleted them all. I wrote 3 pages of the same paragraph with slightly different wording each time in an attempt to get it perfect. My outline (for this 6-8 page paper!) is 7 pages long. But I have no paper, and I have other major midterms coming up that really just compound my panic over this.
So my question is: how do I talk about this issue with the professor? I want him to know that I'm not trying to take advantage of him or slack off, and that I do actually know the material (as my ridiculously detailed outline shows), but I feel like walking into his office hours and telling him about how my nightly panic attacks are keeping me from writing this essay is...inappropriate? Should I mention my mental health issues at all, or will that seem like some sort of weak excuse?