as salam alaikum
Indeed Allah is the Doer of whatever He wills (11:107). At the same time He has taken upon Himself mercy (6:12). If repentance is done sincerely and with remorse, and the promise to God not to sin again is accompanied with true resolution, Allah is the All-Forgiver.
However if some duty has been missed (like prayer or fasting) or if the rights of some people has been violated (like in the case of stealing or backbiting), the person should also make-up for those errors and/or adjust what has been violated.
With prayers for your success.