My only adult child's lack of financial wisdom scares me. How do I protect my legal and financial affairs?

The worst thing anyone can do is to hand over their financial future to someone just because they are "family" and may be offended if they are overlooked. This is the time for very sober thinking and non-emotional planning. Do not hand over the reins to a child who has poor financial or investment skills. Choose someone who both knows you well and whom you can trust.
Mom's financial planner helped her create a trust. I am her trustee. This plan protects her assets. I do not have any legal rights over her assets, I am just here to do as she has instructed. The key issue is finding someone whom you can trust with your financeds in case you are incapacitated. There should be no shame or guilt in deciding that your child is not that person. Not everyone has the same talents or knowledge

Sounds like you need someone you can trust with your finances other than your kid. Probably won't be a very popular with them, but what other choice do you have?