Everyone makes mistakes sometimes (see my post yesterday), and mistakes will happen in your financial life. It is unpleasant, often expensive, and most of all, utterly inevitable. Here's the important part: looking at them, seeing what you need to learn or change, and then letting it go far, far away, never to haunt you again.
What do I mean? Surely there has been some time in your life where you've messed up with your money. Whether it is the medical bill you had to pay because you didn't get it properly cleared through Tricare first, or the late payment fee on your credit card, or the snazzy new car you paid too much for, mistakes happen, even to really amazingly savvy people. Of course, no one wants to make these kind of mistakes and we're all striving to avoid them (as we should) but there is a strategy for learning without obsessing or beating yourself up unnecessarily.
First, look objectively at what happened. What could you have done differently to prevent it? (Disregard the actions of anyone or anything else – just analyze your own behavior.)
Second, see how you could change your financial systems to prevent this from happening again. If necessary, set up overdraft protection, find a better filing system, or do whatever else you need to do to make it hard to make the same mistake in the future.
Third, move on! This is hard for me (I'm still stewing over a medical bill from several years ago, one that could have been prevented if I'd followed Tricare's rules in the first place). Time helps, but you can also try more active techniques such as writing it down and ripping it up. It also helps to remember the times that surprise money has fallen into your life – assuming your financial mishaps aren't a regular occurrence, they're probably being pretty balanced out by the windfalls.
Just knowing that no one manages money perfectly should help you feel less guilty when things go awry. If you can look at the expense as tuition in an educational experience, and use your knowledge in the future, it should lessen the blow a little and let you move on.