Calling all personal trainers

[Seajay]

...just regular athletic shoes (and, no, I don't get sore arches).

If you can imagine your IT band flicking back and forth over the lateral condyle of your femur....getting more irritated with EVERY pedal stoke. You can understand the problem.

Stabilizing the foot on the pedal is important.
First by getting rid of flexy (unstable while cycling) running shoes.
Second by providing support to the forefoot. Many of us have a varus forefoot which can "collapse" as we push down on the pedals. This sends the knee inward with each stroke.
It's really not supposed to do that.
However, so many cyclists do it that it is considered "correct form"....It is not.

[KnottedYet]

Since none of us can actually see you on your bike and assess you, we are really just throwing out ideas.

Don't take anything we say as written in stone and correct or accurate.

If you can do it, I'd love to see you get a bike fitting (at a shop) or a bike fit assessment (through a physical therapist, which insurance will cover).

A fitter or therapist will also be able to tell if you have poor posture habits that would require stiffer shoes or medial wedging or targeted strengthening like Seajay mentions; or if you have some other posture or habit issues.

They would both be able to assess your body and the bike, and make sure the bike matches your body's needs. If you don't have poor posture control, and the problem is the bike, the fitter would be able to work on that the best. PTs' licenses do NOT cover changing out parts on bicycles, and anyone who does work like that on the bike will have the separate liability insurance to cover bike modifications. (it doesn't hurt to ask...) More likely a PT would send you to a shop with a list of changes they recommend for your bike.

Either PT or fitter would be a good place to start. Either could send you to the other if they feel you need work outside of their scope.