My gf and I broke up in Aug 2019. We had a big argument over the phone and we both decided to just break up. This happened while my gf was on a business trip with a male co-worker, who she and I knew was a total fuckboy (co-worker went to the same university as us). So long story short, I’ve always told my gf to watch out for this co-worker. She would always reassure me that she’s always careful and that she wont let anything happen Anyway, less than a day after the breakup, my gf gave her co-worker head and started a month-long fling with him. They would make out in their office after everyone else had left and they would go on dates together. It went on for a few weeks until my gf found out that the co-worker had a gf he didn't tell her about. My gf started distancing herself from her co-worker and we eventually fixed things and got back together. I asked her if she hooked up or did anything while we were apart. She said no and we moved on. Our relationship actually got better after the breakup. Fast forward to Aug 2020, the co-worker’s ex messaged me out of the blue and told me about everything. I didn't believe her at first but I confronted my gf about it. My gf only confessed to kissing ONE TIME but I managed to get the WHOLE truth out of her. She even confessed to messaging the co-worker to not tell me anything because she was “desperate” to hide the truth in fear that I would break up with her. I am obviously hurt by what she did immediately after our breakup, and the fact that she lied and hid this from me for a year. She claims she was only really lonely and lost at the time when she initiated giving head to her co-worker. She told me she just liked the attention he gave and that she wanted to do something rebellious out of anger over the breakup. My gf also reassures me that we got back together not only because of the revelation that the co-worker had a gf (she says this only opened her eyes that what she was doing was wrong), but because she had always loved me and wanted to fix things right from the very beginning. I broke up with her after I found out. She bawled her eyes out and was literally at my feet begging me not to leave her. A few days passed and I forgave her and decided to take her back. She swears she would never lie again and that our relationship would come out stronger after this. Should I believe her and stay in our relationship?