I think you should never make a promise when you are not 100% sure you are going to keep it. Even if you’re happy or angry, but especially then. Because when you will be calm and rational you’ll see things in another perspective and then you’ll regret it. So, from now on you should keep this in mind.
The best thing you can do now is to admit that you shouldn’t have done that and tell that person the truth. At least you talk about it than just break your promise without saying anything and disappoint that person. If I were that someone, I’d still be disappointed but not that much, and I would appreciate that you had the power to admit that it was wrong to promise that.
Everybody makes mistakes. We are human.