I often spend time rationalizing (or de-rationalizing) ownership of things that I know were not Mustachian purchases. Key word there is ownership--I already know the decision to buy was not entirely rational, so that's a moot point. Then the issue becomes a question of whether returning, selling, donating, etc. is the rational next step, or is ALL the financial damage already done and nothing could be gained by getting rid of it.
If you bought a car that gets 20 mpg and you now realize for the same money you could have gotten a car that is exactly equal in every way (price, quality, safety, utility, etc.) that gets 22 mpg, it's not necessarily rational to sell your car and buy that other one. Yes, it would have been more rational to buy the more fuel-efficient car in the first place, but if you don't log a lot of miles, it might not be worth it to incur the costs associated with the purchase (tax, tags, title, etc.) until a few years down the line when you were planning on making another purchase anyway.
Chalk it up to lesson learned and move on.