I know others will reply, and they will have good suggestions for you again. I don’t really understand why Apple Mail is not working. I’d bet it’s not Apple Mail itself. Try sending to yourself. But also take up the suggestion of @TonyTownsend and start a Gmail account. If nothing else now you can use it for testing to ensure Apple Mail is working. After starting the Gmail account, again either send an email to yourself at this new address, or ask a friend to send to you at this new Gmail address.
Again, Apple Mail should be working. I use Apple Mail (and another email app), my wife uses Apple Mail, and we both work on MacBook Airs. I’ll bet you a cup of coffee when you open a Gmail account and set up the outgoing (SMTP) server address, Apple Mail will work just fine for you.
You say your Comcast email is your user name on many websites, and ask if you can keep using it as a user name. I think (and others will have good answers too), the answer is yes, with some understanding. Often a user name is just that: a user name, that allows you to log into a site. It doesn’t necessarily mean email will be sent to that name, although it might, depending on the site. For example, one of my retirement accounts uses one of my email addresses (see what I say below) as my user name, but email is sent to a different email address. In other words, the user name is just a name, even though it “looks like” an email address (e.g., xxxx@XXX.edu).
So yes, you can probably keep using your Comcast email as a user name on these sites. Also, if you want, for important sites (banking, etc.) you can very likely change your user name (and password, which is not a bad idea to do occasionally anyway).
Also, remember, if you follow @TonyTownsend’s advice, and you should, eventually you will want to auto-forward your comcast email to the new email address (probably Gmail). This is in fact, what I do. Not with Comcast. I have a university email address. It’s reliable, that’s not a problem for me, but for other reasons I started my Fastmail account years ago too. Mail sent to my university email address is auto-forwarded to my Fastmail address (and it remains on my university account as well).

