What does Mrs. Mallard learn?
I'm not sure she learned anything. She did get a chance to express feelings that had probably been part of her life for a long time ... and give us the chance to experience them with her. It's possible that she learned something through externalizing what she was feeling, but I don't think there is enough evidence to substantiate that ... and I don't think the story really has anything to do with what she learned.
But there is something that she might have learned. When she walked down the steps she did learn that her husband was alive and that all her dreams for the future were gone --- and perhaps she learned that life with him wasn't worth the effort --- but that's just a guess.
To me, what Louise Mallard learns is that she is a person in her own right. She is not just some extension of her husband -- she is her own person.
When she hears of the "death" of her husband, Mrs. Mallard is overcome. At that point, she thinks that she cannot possibly have a life now that her husband is dead. This shows that she is thinking of herself simply as part of him.
But then she comes to realize that she is actually looking forward to the rest of her life. This means that she must have learned (or come to realize) that life on her own, as an individual, would be worth living.