Hang on. The Apple ID is just that, an identification of your Apple account.
But because Apple encouraged everyone to use their email address as their Apple ID, we each used the same characters (i.e. <my name@email provider dot com>) as our Apple ID. Unfortunately, when a user changes their email address this confusion is what happens.
Normally, one would use a different password for the Apple ID to the email as they are two separate accounts. So the first question is did you use a different password for your email address and your Apple account (you should have done, but I suspect many people didn't because of this confusion).
Think of it like this; the email ID is an identifier, and the same identifier has been used for two completely separate things:
- in the UK, many towns have a road named "High Street". So it is possible that while I might live at 47 High Street, Sometown, my friend might live at 47 High Street, Anytown. If I am talking to friends in Sometown and I would tell them that I live at 47 High Street, but I would not mention the town since that's where we already are. But if I was in Anytown, or if I was telling my friend from Anytown where I live, I would say 47 High Street, Sometown
- I know that in many areas of the US house numbering continues on from the previous street. But the same principal applies; there may be the same street name in two separate cities and it's possible that the same house number may occur in both cities.
Back to your issue. You should be able to use that same ID as before (i.e. your old email address) and the same password as you have always done. Once in your account, you go to your account details and change the email address that Apple can use to communicate with you.
Finally, when you bought music from the iTunes Store, the account that you used to buy it was that specific Apple ID. If you create another Apple ID and sign in via that new one, you will have no access to the music purchased with the other account. So to gain access to music purchased with your old account, you need to sign into that account.