I used to speak fluent Spanish, but now I cannot pick it up again.
Children learn languages readily because they have more open brain circuits than do adults.
Also, they can unselfconsciously immerse themselves in the learning process. I'm no language expert, but I'll bet your Spanish would return pronto if you were to throw yourself into a Spanish-speaking environment, such as a neighborhood, foreign country or language lab. That's true of any skill. Attitude is crucial. A century ago, women believed that they were more attractive if they bound themselves in corsets.