Did you actually wear the helmet to ride in? My recommendation when I sell a helmet to people is, if there is any doubt whatsoever about the fit, to always wear it around the house first for at least an hour to see if it fits or causes any sort of discomfort. If it does, then you know it isn’t quite the right fit and you should return it. If you got it at Dover, their return policy is right there on the website:
[INDENT]Shop with 100% confidence at Dover Saddlery. If something doesn’t fit you or your horse, return your original purchase within a year, and we’ll make it right with a refund, exchange or replacement.
Our experience as riders and horse owners ourselves is that in most situations a year’s time is fair enough to evaluate proper fit and function. PLUS: For manufacturer’s defects, we’ll extend the manufacturer’s warranty to a maximum of 24 months, regardless of their individual policies. (AND: Manufacturer warranties longer than 24 months will be honored and supported by us as well up to the length of those warranties).
Exceptions: Safety items such as helmets and protective vests may only be returned within 30 days, with their original tags intact. Saddles and riding boots may be returned within the one year period, but are subject to a prorated refund based on usage. Used saddles may be returned for 30 days only. For everyone’s protection, treats and vet meds which have been opened are not returnable, and returned blankets must clean and free of debris.
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ETA: Sorry, you posted as I was typing up my response. I would try to sell it privately; maybe it will fit someone you know? You may have to take a hit on the price since it’s used, but at least it’s something back in your pocket.