I really have to be careful also to get a helmet that doesn't put pressure on my forehead. What motorcyclists call a "Shoei head" (except the Arai people, who call it a "long oval" ).

I'm good in my Specialized Propero. Giro doesn't fit me at all. I used to have a Trek helmet that fit me pretty well too, but they don't make the shells in as many sizes as they used to.

But +1 on trying on several different brands and styles - and also letting the LBS help you with the straps, which are much trickier to adjust than they used to be IME. Maybe you can combine the trip with another errand.

My personal opinion (which may not be popular) is that you're safer with no helmet than with one that fits so poorly. If it slips around your head it won't protect you in a fall, and it could possibly lead to a neck injury.

If you really have a small head, you may still wind up in a youth helmet. But the LBS should have some without Scooby-Doo.