Yay! I'm sure you feel much better knowing things are back to rights. I can totally empathize with the late-payment thing. I've missed a payment for stupid reasons, too (credit card expiring and neglecting to update all accounts). What's more, this actually happened more than once. I was so angry with myself! Plus, it was really hard to put it out of my mind until I was able to get it resolved. On the other hand, I've also been on the other side of things -- my tenant was once a bit late for similar stupid reasons. I was actually slightly relieved to find out that other smart, responsible people make mistakes, too. And no harm was done.
More generally, I'm trying to teach myself a bigger lesson: when I make mistakes, I fight the urge to stress about them. Seriously, stress can and will cause damage to one's health and one's relationships. It also doesn't redeem any of one's "failures." Instead, I try and correct the issue at the earliest possible time and take steps to minimize the chance it will happen again. I like your strategy of setting email reminders, and I think it also helps in cases where I can't do anything about the problem immediately (if it's, say, a weekend). Often, just knowing that I'll be reminded to fix the problem when I can actually do something about it helps me switch my mind over to other more productive things *.
* Credit to David Allen of "Getting Things Done" fame for this trick.