Can I grant POA to an agent on things like major purchases, where I live, marriage?

I agree with AlvaDeer that you should go to an elder law attorney to see what all your options are and how to protect your assets to ensure no one can ever be tempted to abuse them (or you). Often financial abuse of the elderly is a crime of opportunity committed by people you would least expect...including family and friends. Just read some of the stories posted on this forum under Elder Abuse. An attorney can help you do some wise estate planning, like creating a trust, so that you are thoroughly protected. When it comes to access to other people's money, you just can't be too careful...or trusting.

You can grand POA to anyone you like for both your health care needs when you cannot make them, and for your financial needs when you cannot carry out your own purchases. But this person must be meticulous in keeping records and spotlessly clean and honest as they can take advantage of you otherwise. Have you family that you can trust? You say you are a poor decision maker. Can you not then just ask the advice, counsel and help of this person? I am all for having a strong POA, but you say you are not good at decisions, and this is ONE HUGE DECISION. Wishing you good luck. Hoping you tell us what you decided and how it went.
No matter it seems expensive and a waste of money, get the counsel of a lawyer in drawing up these documents.