I speak at conferences a lot and sometimes, after I say yes, I have a little "buyers remorse" and I feel nervous about it and I just want to back out of the whole thing. I never have backed out though, and I have rarely regretted going.
In addition to the pros listed in other answers, being able to list a conference where you gave a talk has value on your CV even if you gained nothing from the conference itself. And your con (my supervisor will be upset if I cancel) is also a pro: I will please my supervisor by going. That said, you can and will get a LOT from a conference, any conference, if you decide to.
Grasp firmly onto a positive attitude. You have three weeks. In that time, don't just prepare your 15 minute talk, prepare your plan for the conference. This includes your plan for the travel and minimizing your jetlag. (You might be interested in a sister site for tips on sleeping on the plane and avoiding jetlag.) This also includes a list of goals. Say I was going to a conference of geologists in Paris and speaking on how the name of your cat affects your income. I might make a list like:
- go up the Eiffel Tower
- meet a geologist who lives and works near me
- visit the Louvre
- find 3 other cat-related or income-related sessions at the conference, attend them, and introduce myself to the speaker
- achieve an attendance of at least 50 people at my session
- record my "speaker rating" and ranking and report it back to my sponsors
- buy two bottles of wine to bring home
- discover at least one aspect of geology that is relevant to my cat-naming research and learn enough about it to summarize it to my colleagues when I return
- eat warm croissants in a park
- write up a one page summary of who I met, what I learned, who now knows about us and our research, and some opportunities I will pursue when I get back, and give this summary to my sponsors within a week of my return
Then I would keep this list of goals handy and push myself to make the necessary plans in advance and take the necessary actions during the week to meet the goals. This includes looking over the session lists as soon as they're available, planning what talks to attend (and when you are likely to have a free morning or afternoon for sightseeing and croissant eating), emailing other people who are attending to arrange to meet them there, and so on.
Making the best of a conference trip is a lot of work, but the rewards can be substantial. So many people have to beg and plead to even attend, and here you're being flown 20 hours away in exchange for only 15 minutes of talking! You must be doing something darn interesting.