How can someone be so undecisive

i used to talk with a girl in 2023 and after few months of talk i started liking her. our conversation used to very deep, therapeutic, we shared everything about each other's day, mostly online, just met once in person. i started liking her and confessed about it but she refused to accept and we stopped talking after few months. the ending was very messy and i thought we would never talk again yet i kept trying to reach her out. after 1 year she came back saying she missed our conversations but by the time she already started seeing someone else(her bestfriend ofc). stupid me still accepted her and decided to control my emotions and not let her take the control because we both were preparing for an exam and i thought it would be better to have someone i know to talk with. since last year everything was fine, we became good friends. few months ago she confessed that she used to like me back then too when i confessed but she didn't said anything and she is now feeling trapped with her current bf. stupid of me believed everything she said. we started talking more,sexually too. but after one month when we were about to meet again she out of nowhere met the guy she felt trapped with and they reconcilled their relationship and she realized she only loves him and wants to let me go. i was so angry and saddened about this and decided to cut her off but she kept manipulating me emotionally untill i was convinced (which i was , stupid me) but then within few days she started guilty about how she cheated on her bf with me and started blaming me and herself blocked me.
she gave me a glimpse of what i was craving and somehow blamed me for signing up for this.

i'm really in a very bad shap processing all of these, just need some outside perspective so as to let go of her permanently or should i accept her if she ever tries to come back?