I need to vent because I found out that my girfriend has been having an EA with her ex girlfriend from highschool.

At the beggining of our relationship, she shared with me that she had a "best friend" and this best friend happened to be her ex from highschool. We are into our mid 20's so at first I decided not to be threatened by a past fling and enjoy our connection. 4 months later, I notice my girlfriend is VERY emotionally detached from me, not sharing anything out of the norm, and I had to pry and beg for her to share mostly anything at all.

2 weeks ago, her "friend" called her and I noticed the girl had a loooot of emojis by her name, so I answered the call.

My Gf took the phone into the other room and pretty much told her friend that she couldnt talk she was busy. I asked her why she didnt stay on the phone with her friend and she brushed it off and said she'd call her "later or another time" I now realize that she didnt want to speak with her while I was around so she could express herself freely with this girl.

She was hiding to talk to her "friend" and deleting messages between her and this "friend"

Whatever it was, I didnt like it and I told her to promptly cut it off. I told her if she didnt want to cut it off, she would have to stop sneaking around with her.

Fast Forward to This week, I began getting suspicious of their involvment so I went through her phone and saw that my gf was sending this girl good morning texts, hot sweaty gym pictures, and flirting in general. I was able to read a text where the "friend" called me "demonic" which leads me to believe that she has spoken negatively about me to this girl, so the girl felt comfortable openly disrespecting me to my partner.

She goes to work that day and when she comes back, the messages are gone. I ask her why she deleted the messages and she says she did so to protect me. However with some more truth seeking and information seeking, she was finally able to confess that she was physically and emotionally attracted to her the entire time. She opened up about how she had shared things with her "friend" before sharing them with me, she also opened up to not giving up the hope that they are both single at the same time so they can finally together.

She told me that in highschool, they made a plan to move in together after college but I guess it never happened .

Obviously I am feeling very betrayed, hurt, and angry. At this point, I'm trying hard to forgive her, but she acts like nothing happened, she says the friend is "blocked" on everything, but if she can delete messages, and sneak around to talk to her when she knew I didnt feel comfortable with this interaction, how can I know that she is blocked for good? Can I rebuild trust in this realationship and how does that look like? Anything helps thanks!