wouldn't let me use my old id to download my music
If you still know that 'old' Apple ID, check your Apple ID via your email box.
See this Apple article: If you forgot your Apple ID
If you forgot your Apple ID email address or phone number - Apple Support
OR see -> Having trouble signing in?
https://iforgot.apple.com/password/verify/appleid
See -> If you can't connect or sign in to iCloud
If you can't connect or sign in to iCloud - Apple Support
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Apple says:
Your Apple ID is the personal account you use with all Apple products and services. It includes the email address and password you use to sign in, as well as the contact, payment, and security details that you use with Apple.
You can’t merge multiple Apple ID accounts.
See the following Apple articles
Sign in with your Apple ID -> Sign in with your Apple ID - Apple Support
Manage and use your Apple ID -> Manage and use your Apple ID – Apple Support (UK)