Check via your local library system if they carry the book "Fixing Your Feet" by John Vonhoff. I suggest reading up on P.F. there, and you might also just read through this in general, things like preventing and fixing blisters anyway.
Consider going to an actual foot doctor and getting diagnosed and then likely getting custom orthodics. Not cheap, but ... might be worth it (so very hard to know for sure). I find that a pair of custom orthodics last me well over one thru-hike. After closer to two thru-hikes I ended up having worn through the underlying hard plastic part with holes forming at the heels and cracking the plastic. For more typical use the underlying hard plastic part lasts a long, long time and you can just optionally resurface the soft rubber upper on occasion. Even buying a replacement orthodic completely was cheaper, however, since I still had the molds.
As far as picking a good shoe --- the more you know about your own particular feet, the better. I can now have pretty good luck buying shoes online, just finding sites that give various views of the shoe, and particularly looking from the bottom up at the sole and vice versa, top-down to try to guess at the toebox room. And reading the reviews. Zappos.com is a good place to look for data, and if I recall correctly you can send shoes back to them for a full refund if you've only tried them on inside your house, not out on the street. They won't always be the cheapest source (and if you end up buying 8 pairs at once as I did last time, you do take the time to look for best price). But zappos did a fine job for a friend in doing fairly rapid on-trail delivery to a post office, they seemed to know how to work with thru-hikers FWIW.
Once you recognize that the authoritative-sounding guy in the shoe store might not know all that much that applies to what you're doing, IMO it's ultimately better to "shop" at an online store that has a much much larger selection, even if that turns into an iterative process that takes a while. You really need to become your own personal foot/shoe expert to some degree I think. It's always possible that you'll find a shoe store with the magic combination of a shoe salescreature that really listens and really knows a lot AND where there is at least one pair of decent footwear in your size that works for you. Or, invest in lottery tickets ...
Best of luck with this!