how to trust and forgive my girlfriend after i caught her cheating ?

need some help guys, i 24m have been dating a girl 23f from my high school, we were friends back then. she dated a guy during her high school (the same high school stupid love),we went to different universities, during college we were not in contact for almost 3 years, i dated another women but we did not have sex. meanwhile she dated another guy (for a very short time). after we got out of college we met almost 4 years later and kind of started talking. soon we came into a relationship and its more than 1.5 year now. she said that she slept with both of them but it was her biggest mistake of all. if only one thing she would like to change is her past. now we moved together ofcourse i was the one paying for everything. one day i went through her phone and found she was still talking to her ex while we came into realtionship. but she stopped soon. and from that point her never texted or came into contact with him. it was just after 2 months when we came into relationship. i was pissed af , i confronted her and all she could do is cry and begged for forgivness and said her ex was suicidal that's why she had to answer his texts. but why did not she tell me?
after that incident i am not able to trust her anymore, even if she begs all the time that it was a mistake and i stopped myself because it was wrong , i just can not trust her. i was already going through retroactive jealousy when i found it she slept with both of them ? and now this , it just fucks my mind.
even if it was mistake and she did not want to cheat on me, i feel betrayed , she says all the time that she loves me but i am just not able to trust her anymore . help me here.