OP says her gut feeling has been there a long time.
That would drive you insane.
Look, I'm as scathing, sane and sensible as the next person about the bollocks that is mediums, but years ago after years of unsuccessful TTC, it crossed my mind "if they could just tell me one day... to keep me going..."
What I'm saying, is years of stress does silly things to you.
OP, you might be a loon :) But if you've had years of suspicion, your head will be all over the place.
Now, these mediums feed on what you give them. Very clever at it too. So what you fed her, is stuff that really worries you - like, the OW could be a dog walker. There must be something in your H's dog walking behaviour that worries you.
If I were you, I'd clear myself some time, and write down EVERYTHING that makes you fear an affair. Some people are just paranoid, but I think most people having a good gut instinct. Listening to yours will tell you where to look.
If you think he goes out to call an OW on these frequent shopping trips - check his phone. Listen to your gut.
The medium has no more idea than I do - but that doesn't mean it has been a wasted exercise. You can't go through years more of suspicion, it's a horrid way to live. x