Changing my email password on Xfinity caused Apple email unable to work. HELP!

Thanks for replying, BDAqua. All of my software is up to date (OS), and I spent about 4 hours with an Xfinity tech (a "level 2" tech rep) and about 3 hours with an Apple tech supervisor (he told me there is no one higher than him in a tech-support position). Neither of them were able to solve the problem on my iPhone, MacBook Pro, or iMac. Both were completely baffled and basically threw their hands up in the air after walking me through the set up multiple times. The Apple tech couldn't screen-share with me (some problem on his end), but yeah, no resolution and in the end, they both just pointed their fingers at one another ("It's Xfinity's issue" and "It's an Apple Mail issue") as to what the problem was. Drat :(

Thanks, BDAqua. I fear that you're about 77 levels above me on Apple/tech knowledge (perhaps even 78 levels! ;), so you lost me a bit there. I opened the "Keychain Access" window but I'm uncertain what you meant here:

"Enter the part after the @sign in the search bar, then either check those Passwords under those items, change them, or delete it and start over"

I have deleted the email account numerous times when on the phone with tech support and attempted to sign in again, but I get "Unable to verify account name or password" as my error message. They even had me go back through the XFinity site to redo my password to make certain I wasn't entering it incorrectly or something, but alas... no luck. Honestly, I changed my password about 5 years ago and helped my wife change hers last year and it was NOT a problem whatsoever. I am (along with 2 evidently well trained tech support folks) completely at a loss. Perhaps you're onto something with the Keychain Access route, though again, I'm not clear on what you're suggesting I do. Apologies for me being daft and I do appreciate your attempts to help!