This is such an important skill, personally as well as professionally.
We do the value assessment as consumers all the time. “is the benefit this coffee offers me worth more than the $4.50 I’m spending on it?” The next time you want something, focus on what that means for the other person. It’s not even just money: they may be concerned that you’re asking for time off to attend the conference. Anticipate the trade-offs the other person has to make, and be prepared to have an answer to their objections.
Doing this sort of preparation will help you feel much more confident that what you want is actually in the best interest of the other party, you should have less fear about asking!