how do I fix this? Somehow I have the wrong great grandmother on my paternal side and that has created generations of errors. Can I start over and do a new Tree?
It's a single, collaborative tree, so no, you can't "do a new Tree". What you can do is delete the incorrect relationship(s), and replace them with the correct ones.
Here's a Help Center article on correcting relationships in the tree: https://www.familysearch.org/en/help/helpcenter/article/how-do-i-correct-parent-child-relationships-in-family-tree.