Is going to a mosque that one's mother did not wish for one to go to considered disobedience to her, and how can one repent from that?

Obeying one's parents is obligatory unless they command disobedience. You must obey your mother if she forbids you from praying in a specific mosque and tells you to go to another. If you disobey her, you must repent and seek her رضا (satisfaction). You may gently discuss the matter with her if it is difficult for you, but if she insists, then her obedience is obligatory. If the forbidden mosque is close and you fear missing the congregational prayer by going to another, then pray in it, but do not inform her of that.