Thank you, everyone, for your responses! Fortunately, all of my recommenders submitted their letters today. Yesterday, life seemed pretty bleak, but now I feel much better.
I left out an important detail. My recommenders are not professors. Unlike professors, my recommenders do not carry the same kind of heavy workload, they do not have countless letters to write nor are they familiar with the process. I made sure to ask them if the target date was fair, if they had any questions or concerns. I gave them all a lot of guidance on these letters. They all said they would submit by my target date, that it was "more than enough time." If they wanted/needed more time, all they had to do is tell me. I'm very understanding and considerate. After all, they are doing me a favor. But they never hinted at that whatsoever. Then they didn't respond. That was my issue.
To give a sense of one of my recommenders...Today he said to me, "Maybe if I put these off long enough I won't have to do them at all." He was not joking. Shame on me for choosing this character. I immediately removed him from sending letters to my top two schools. I gave him plenty of opportunities to back out so I just don't understand.
I set artificial deadlines for multiple reasons, but as I think about it now, I think a major reason is because I didn't trust my recommenders enough.
You live and you learn, I guess. And boy, I have learned a lot in these two years after undergrad.