What is the ruling on the actions of a committed young woman who agreed to marry an already married man much older than herself, then regretted her acceptance due to his words and actions that are impermissible in Sharia, and how can she atone for her sin and avoid repeating these mistakes?

What you did, speaking directly with the man or over the phone, and what happened with him holding you and your meeting at the bus stop, contradicts Allah's saying: "And tell the believing women to lower their gaze and guard their private parts" and His saying: "then do not be soft in speech, lest he in whose heart is disease should covet," and the Prophet's (peace be upon him) saying: "No man is alone with a woman but that a mahram (definition) is with her."

You must hasten to repent by cutting off this relationship immediately, resolving not to return to such a thing, and regretting what has passed. Sincere repentance erases all sins.