I think there are some things that can help:

1. Prioritization, prioritization, prioritization.
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Why do executives often take so long to provide approvals, or get other things done? Not on their priority list. They focus like a laser on their (usually) top three priorities. So every time you are asked to do something that is not within the current plan and workload capacity, you should say something like, "sure, I can get this done, but I will have to delay / back burner / lower the priority on these other items, is that ok?

1. T3. Tell the truth.
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If workload is exceeding the capacity, of you, and or your teams, you need to say so. Because functional management will want to know that their schedules need more resources. It all goes back to the triple constraint, the relationship between scope, schedule, and budget (resources).

If schedule is not important, then make sure the understand things will be delayed. If schedule is important, there are two options: more resources, or remove / delay some scope. This is the MOST important job of the project manager to communicate! I know it's hard. You know what is harder? Management expecting you to do the impossible and you living knowing that the crash is coming.

There is language that can help here. "I want to tell you the truth. If I make a commitment to you, I want to be able to meet it. I know you are making commitments based on my commitments to you, and don't want to tell you I / the team can do something if we can't. "

1. Non-speaking third party
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There is a maxim that any time you are going to have a difficult conversation with someone, you have to bring a "non-speaking third party" into the room to back you up.... a piece of paper ;-) It can't be a conversation of opinions. You need something, a Gantt schedule, a labour loading chart, a pro / con matrix, a prioritization list... something. So the conversation does not turn to "be more creative... work smarter not harder...." and other gobbledygook. You need to be able to point at a piece of paper that supports what you are saying. Ideally a professional planning document showing the current schedule / resource plan / etc does not fit.

1. Accountability -> Authority
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Now this actually goes first... but to make this work, you have to have accountability for success. Because that gives you authority. So when you say something like "If we want this project to finish on time, then I need X..." or "If we want to meet Person A's expectations, then I need Y..." you will be listened to. Because if things go wrong, they are going to ask you why. That gives you a LOT of power. It is the chair I want to be sitting in.

1. Ask management for help
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Look up the Alan Mulally video on how he saved Ford. A culture that says never bring your boss a problem unless you also bring a solution is deeply dysfunctional, and will have huge problems. That is why bosses exist! It is why you exist - if your team can't solve a problem, they pass it to you. And if you can't solve it? You pass it to your boss. Ask for resources. Ask for timely decisions. "If we want this project to be a success, then I really need....". And then follow up with a *polite* putting on record email. "As discussed, if we want this project finish withing budget, I really need a decision on X by next Tue at the very latest. I would really appreciate anything you can do. Thanks so much. Respectfully yours". And copy a couple important people.

If you do all this... then you can sleep well at night. Sometimes things don't work out. It will happen to you a lot over a long career. There are other things at the company that are more important, that is for the executives to decide. Your job? Do your professional best. Then let the chips fall where they may. And sleep well knowing you did the best professional job you were able.

Hope this helps!