Different Accounts for iTunes and iCloud

Ok, here is my problem - hopefully someone can help.

I have an Apple ID that I set up years ago and used for iTunes before it became mandatory to have @me or iCloud accounts.

This ID purchases all apps etc.

My iCloud account was set up using my @me.com address

When I set up family sharing, I can't authorise requests as family sharing has to use the @me.com/icloud account and not the Apple ID I use of iTunes.

This also causes me issues in creating additional iCloud storage as it won't match up across accounts.

What can I do to solve these issues?

Hopefully this makes sense!

Thanks for any help provided