How do I get people to accept an honest answer they don't want to hear?

You can do the work now. The part that still defeats you is human, not technical: telling people something true when they were hoping for something else, and watching them push it away. That resistance is hard to sit with, because you did your job well and it still was not enough. But their pushing back is rarely about your numbers. It is about what the numbers cost them. People do not reject an honest answer because they doubt the math. They reject it because accepting it means changing a plan, admitting a mistake, or letting go of a hope. If you argue harder on the facts, you miss where the real resistance lives. Meet the feeling first, and the facts get a fairer hearing. So before you deliver the unwelcome answer, name what it means for them out loud. Acknowledge the disappointment, then stand quietly and firmly by the truth. You are not there to make them like it. You are there to give them something real to build on. Deliver it with respect, and let them come to it in their own time. Some will. That is the most you can honestly do.