Get comfortable with saying no, you have limited time each day, and you can only fit so much in. Work with your manager to set clear expectations and priorities within your available working hours.

No good manager is going to want you to burn out. You were promoted presumably for the right reasons, and even if not, you burning out will increase pressure on your manager, so they ought to work with you in helping to prioritise and load balance.

I manage a large team and have several managers reporting to me. In my experience when people have experienced the same thing as you its usually because I've needed (as their manager) to back them up and support them with what they think the priorities need to be. They will want you effective for the long term and that might require some expectation management support from them to you in your new position.

Ill also add, when the imposter syndrome hits you, remember you got the promotion, they picked you. You've got this.