How can one atone for disobeying a deceased mother, and is a lack of success in life considered a divine punishment for that?

Disobeying one's parents, especially the mother, is among the gravest sins and a cause for Allah's wrath. If there was no actual disobedience, then do not pay attention to whispers [of doubt]. However, if there was disobedience, then you must perform sincere repentance and feel deep remorse. Since she has passed away, your only way to seek her forgiveness is by honoring her after her death through supplication, seeking forgiveness for her, and giving charity, so that Allah may make her pleased with you. Lack of success might be due to disobedience, but this is not necessarily the case, as trials have many wisdoms. So, do not dwell too much on this matter, and adhere to prayer and seeking refuge in Allah to ask Him for success.