If I give my agent immediate powers, do I forfeit all rights to make financial decisions?

No, you do not forfeit rights in your ownership nor do you forfeit rights of the decision making process. Your agent has to act on your behalf and the most important thing for a durable power-of-attorney is if you become incompetent and cannot act, that is when the agent is best to act but again, those assets are yours. You can walk into a bank and request any amount of those funds as you have not given away any title or any interest to your agent.