We've all done it before-- no matter how much you're on a budget, no matter how frugal you're being, no matter how many Ramen noodles you eat instead of going out, there's that day when you sit back, go over your personal finances and say...
I spent HOW much??? On THAT????
Yes, yes you did.
First, learn from it. What item could you simply not live without? Did you make the decision to buy it actually in the store, or before you even went in? Before you even left your house? Next time, is it easier to just not go into that store? Or go grocery shopping with less cash in your pocket, and no cards? Or is it easier to ask someone else to pick something up for you, so you spend less?
Everyone's got their different trigger points. For me, I learn to avoid bookstores if I don't want to buy books. It's just a lot easier than listening to all my convincing arguments about why I need a particular book once I'm in the store.
But everyone's got different buttons. It might be easier to not go shopping if you're not reading ladies' magazines, or it might be easier to spend less on food if you're not watching the food network.
If you have kids, you might find that your kids ask for fewer toys if you don't visit the toy section, or if you mute the commercials while watching TV.
These might not work for you. Experiment.
Next, remediate it. Can you re-sell the item on eBay? Can you return it at the store? Can you apologize? If so, consider it. If not, get over it.
Now, move on. Get over it. Quit bemoaning the fact that you now have something off eBay that you didn't really need or want and was damaged in shipping, and go do something else. Something that distracts you.
Maybe even treat yourself, for learning an important lesson. Not at the coffee shop, not at the bookstore. Find a pleasant place to walk. Find a nice hillside with grass where you can sit and feel the breeze. Go to the library and find a new author. Read something interesting online. Take a hot bath.
And just don't make the same mistake again. If you're treating yourself well in other ways, you won't need to.