AITAH for being mad at my fiancé over something 5 months ago

For context me and my fiancé have been together 1 year and engaged for a month or 2, I am 17F he is 18M. A few months ago me and him went through this really bad arguing phase because I had found that he had been lying to me about a few things. One of his exes and my old joint mates came running to us because she needed help getting out of a long-term relationship with a guy. I agreed to help her and she got out of it and me and him were arguing some more and it was evident that it looked like we were going to break up. She went up to him later that day after leaving her boyfriend and started flirting with him apologising for leaving him a whole bunch of other things. Some important information that is necessary is that from the start of my and my fiancé‘s relationship we agreed that we would have zero exes added as a trust and a way to prevent arguments.

No, he’s where the problem begins because of that rule. He hid it from me that he had her added. and I only found out after doing some stalking on his and her profile and just flat say to him are you messaging her? He told me yes I asked him to show me the messages because he has never had an issue with showing me anything on his phone he would leave it unlocked around me and if I just put out my hand for it he would give it to me. This was the first thing he had hired from me once in our first 5 to 6 months of dating. he refused to show me and got very mad at me and started yelling at me so I gave up on the topic and we just kept arguing that night and we ended up breaking up the next morning. We fixed it and agreed to attempt some new things and well it worked because we’ve now been together for this long.

Fast forward to last night it’s been eating away at me at the fact that he wouldn’t. He wasn’t able to show me the messages. I asked him about it and I had a questioning him on it. He got mad and I just said they just tell me what was in them and then I proceed to find out that he had heard that they were planning to catch up and meet up and that she was continuing to flirt with him And that they met up behind my balcony. He lied about hanging out with a friend. Here is where I might be the asshole, cause I am livid about it. I am so mad. I told one of my friends and she got mad saying that I don’t have a right to be mad because it was four or five months ago and that I should just give up am I the asshole do I have a right to be mad.