Q: I know that I am slipping. What can I do to prepare for the day when I cannot handle things?
A: The first and most important step to take is to do what you just did - acknowledge that you may not have the same grasp and control as before and prepare. I see too many people who are in denial, or insist on conducting their lives as if they were still in their 20's, or simply have no clue as to the reality of the situation and insist on "trying" to understand and to make decisions themselves, regardless of the consequences to themselves, their families, and their property.
Before focusing on what to do for your own legal documents, please consider those persons who may be naming you as a beneficiary, such as a spouse. Request that they set up a trust in their Wills rather than leave property directly to you. That way the assets can be available to you, if needed, but would not automatically be taken to pay medical bills or expenses and would not disqualify you for possible Medicaid support.
Other documents, such as Durable Powers of Attorney, should be changed to remove you as the agent and replace you with someone else. Beneficiary designations should be changed to name the other's estate or trust, not you, as the recipient of the proceeds. That way those proceeds can still be available for your needs without automatically being exposed and taken.
But as for your legal arrangements, you basically have two options. First, if you do nothing, eventually a conservatorship and/or guardianship would need to be established. But this entails an expensive Court proceeding with bonds, reports, and other costly requirements. Additionally, it is possible that a bank or a stranger could be appointed as your fiduciary. On the other hand, if you become rebellious and show the minimum capacity to "handle" your affairs, then the Court will not step in to help.
Your second option is to sign a good set of Durable Powers of Attorney. I like to do all-encompassing documents so that any decision, whether medical or otherwise, can be made. I also like to execute four original documents so that we have some Powers in reserve in case you are not able to sign additional ones in the future. And finally I like to have not only a primary agent but also one or more backup agents in case an agent is not available to act.
You would still be in control as long as you keep possession of the document, only letting the agent have a Power of Attorney if something needs to be done, and then have the agent return the Power when the particular task is done.
But it is important to establish red flags or guidelines with people that you trust and/or maybe with doctors. Thus, when these triggering events happen as envisioned, the Powers of Attorney can be used for certain matters, or even to take over functions, based upon the guidelines that you established.
However, making this approach work means that you must recognize your limitations and be willing to adjust. Life is about changes. How you handle this one is up to you.