how do i stop mine and my daughters iphone sharing imessage?

My daughter and I both have iphones and she uses instant messenger all the time and I don't. She is below 18 so she is on my account although she has her phone. I want to use the icloud but am worried it will merge mine and my daughter's content on the phone and I will start receiving her instant messages again (it happened before somehow) etc!

It's really painful. And I am now running out off storage.

Plus I am also having the problem where apple prompts me to sign into icloud continuously but with my old email address and I have updated my apple account to my new email address and logged on through that but it still persists on prompting me with my old email address and I don't have access to this anymore?

Help

thanks