I made that mistake on the first MyPillow I bought. I chose medium instead of firm fill, ignoring the fit guide, because I was worried that the firm fill would be too hard. (It's just as soft, but much less flat for my heavy head). My 6 year old son got the benefit of that pillow, which fits him better than it did me.

I now have 2 of the firm fill ones and they are great. The best pillows I've ever had. The second one is a backup for when I need to wash the first and wait for it to dry in the dryer, but I normally have both handy and rotate between them.

The fit guide recommends a firmness level based on your T shirt size. Trust the guide.

Enjoy your MyPillow.