College and depression?

Short story: my depression worsened this semester, resulting in me doing poorly in a class. I want to ask my professor for an incomplete or an extension or something, but I'm not sure what exactly to say.

Long story: I was diagnosed with depression last year. I used the free counseling at my college and it really helped. I figured I didn't need any counseling this year (plus we have a limited number of sessions available per year). This semester started off fine, but my depression came back, worse than before. By time I realized that I really needed help, my grades had suffered. In particular, my grade in one class severely dropped since most of the coursework was due during the time of my depression. I am getting back on track, but I don't know what to say to this professor about why my work ethic dramatically decreased and find out if there is anything I can do to save my grade.