Hi everyone, thanks for reading.
Me and my gf have been together for about 7 months. At the 3 month mark we expressed how much we loved eachother and how right this is for us.

Regretfully around this same time there was this male friend of hers she recently established on the side which i had not met and I have still not met. My partner told me that she had an inclination that he liked her but could not confirm it. She confirmed they discussed their friendship at one point fairly recently whether or note there was something more. There wasn't really any confirmation but more of an awkward feeling and no ground rules and they continued to be friends. In a very short space of time they were talking about deep and emotional feelings, sending somewhat flirtatious msgs. Both of them were going through a hard time and they spend a lot of time helping each other out. I was currently in the picture at the time when they engaged in these interactions and I expressed that I did not feel comfortable about it. My partner understood at the time and agreed to tone it down and agreed to let me know whenever they sent phone messages or talked. The first time they were in contact, my partner withheld telling me they spoke and had deep and emotional chats about their feelings etc. I had to ask her and she eventually told me and was sorry. I later found out that my partner went out of my way to not tell him about me and this was over 3 months! She explained that she was trying to protect him from further disappointment and fearing that his current tragic circumstances would be impacted if she told him she had a boyfriend, in case he liked her. I was extremely upset by this. It felt that she went out of her way to make me feel I did not exist. At the time she was understanding of my feelings and made me feel better. At this point she disengaged further contact with him in this way and I was satisfied.

Just a few days ago this issue was brought up and she told me that she felt she was not wrong for doing this because she was feeling depressed and so was he and she felt she had to protect herself and also him so the circumstances were justified. She claims that everything deep and emotional she spoke with him was above board even though she had an inclination that he liked her and also considering she never mentioned me to him. We are now back to square one and I am finding it hard to move on from here and accept that her behavior at the time was ok? Thoughts anyone?