How to disagree without people hating you

Any ideas here? I always struggle with this at work, where I have a professional disagreement with someone (I think we should do it X way instead, I have a good reason for thinking so, it’s something I’m knowledgeable at) but I experience immediate regret as soon as I express the disagreement—I feel like I am jeopardizing our good working relationship or spending out of a fixed pool of goodwill points, something like that.

I guess I struggle with it most in situations where others have already let me have my way on a bunch of arguably unimportant issues, and then suddenly something that actually matters comes along and I am asking for one more dose of tolerance.

When I can tell that others are “just tolerating” me or allowing me to win the argument just so they can move on with their day, it makes me feel very insecure, like I am some kind of narcissist or bully who refuses to hear criticism. But it’s not true! I am more than happy to be disagreed with, and I try my best to express to others that I am open to being wrong—but if others want to be passive and flatter my nonexistent ego then I have no opportunity to demonstrate that I do not take it personally.

My therapist would remind me that a narcissist would already have gotten over it.

How can you be both right and open-minded at the same time? I don’t think it’s as obvious a question as it seems.