What do other people get if you achieve what you want?
If you have a target, and you are committed to it, the best way to approach it is to first figure out what others have to gain. Your colleagues, your boss, the other managers, your company, your stakeholders.
It’s a great exercise to keep your wants in check. Is this really the best thing in this situation? Does it still make sense after so much time? Am I being too selfish, unreasonable, unrealistic? How many people are going to end up better off after I win?
And it is also a way to start thinking how to get buy-in. You always need buy-in, you do not operate in solitary. Having an argument that goes beyond “because it’s good for me” (or any variation of it) is a huge step towards getting it.