Sorry to hear.

I got the same denial letter about 2 weeks ago. I've already appealed it online. My docs too are baffled as to why I didn't get accepted. I'm giving them the 4 areas of functioning (that link that RRU96 gave you) to my docs for their second letters. I meet all the criteria, but I think they don't even read your docs letters or anything. They just wait 6 months and automatically deny you 99% of the time.
My range of life is extremely grim. Can't focus to read books, watch long TV programs, write, or do anything fun. I get nervous when I leave the house so I don't go out unless I have to, like for therapy or my one day of work a week. Of course everyone who's applied for SSI & SSDI knows that they aren't doing it just for fun, it's painstaking and humiliating. Then they basically say it's no big deal, get back to work you lazy ****. It's cruel what they're doing to save some money. If we wind up in the hospital and tax money has to cover it, how are they saving money when they could do preventative measures like medication for so much less?
I'm just as frustrated and upset as you are. You don't have to analyze yourself. We all have functional days, but we have more bad days than good, and you can't work a job when your emotions and physical symptoms are this unpredictable. I'm constantly exhausted from my meds and my depression. Most jobs aren't accommodating enough to give you a week or a month off for an episode. I'm lucky my job lets me come in one day a week and switch the day if I need to, but that kind of income is not enough to live on.
I don't know you personally, but I believe that you're like me. Disabled with some functional days, but not enough to keep a job that can sustain you. Don't give up. They're trying to weed out people who aren't determined enough to continue. And that's what sucks because our frame of mind is so bad after we get such a letter that we wonder if it's worth the hurt to try again. If you need, ask someone to help you with the appeal. I know it's overwhelming, but don't give up.