Hers was a completely normal reaction IMO.
When infidelity happens and trust is broken, whilst many may move forward, forgetting it has ever occurred isn't realistic.
That is the nature and damage that it does.
I would think it very strange for anyone to assert that although their partner had an affair they are 100% certain they never would again.
THAT would strike me as naive.
Of course you can hope that it won't happen again but to be 100% sure? Nope.
Things like serious illness, death, infidelity change a person.
The world and how you view your part in it, your confidence in it, changes.
The affair changed the OP forever, even though she has largely successfully moved on.
She wants to think well of her husband, and does, but that doesn't mean she has suspended her disbelief that her husband is ever capable of having an affair.
History has told her otherwise.
I'm delighted for the OP that her justifiable fear was not borne out.