I applaud you for wanting to disclose to your employer. I can't imagine the amount of courage it would take. If you are in the US and you have been working for your employer for more than a year, you can take a leave under FMLA. You can either leave for a set amount of time or you can request accommodation so that you can come and go intermittently as your illness requires to go to appointments, or if you simply cannot come in due to symptoms related to your illness. If you received your diagnosis from a clinical psychologist or a pdoc you can have them sign off on the paperwork.

You have expressed your challenges with your depression in a very clear and concise manner. You seem like a dedicated and responsible person to write a letter like this. I have never disclosed to my employer, but for me the culture at my work is very toxic and I don't believe that they would understand. If the environment is supportive at your work then I say go for it. You know better than anyone how a letter like this would be received. Whatever you decide, I wish you luck.