Should I be mad at him? Sorry that this story is so long but please read!!

Okay, so, about 5 months ago, my boyfriend and I got into an argument and then he immediately started back talking to a girl he used to talk to and made her call my phone and say she was his girlfriend and stuff, basically harassing me and trying to get under my skin. For almost 2 months, I sat there and watched him flirt with this girl on social media and post her and stuff. This made me extremely depressed. But in April, we started talking again and we got back together and he apologized for the situation and stuff. So, I went to his house in June and he claimed that he doesn’t talk to the girl anymore. But yesterday, I went to his house again and saw the girl’s picture in his phone and then he said she was his cousin but I knew she wasn’t his cousin and he knew that, too. Then I saw in his call logs that he has been talking to this girl every other day, some calls lasting over an hour. There were no text messages, just straight phone calls between the two. I asked him about it but I didn’t really talk about it as much with him because yesterday we were going through a lot of issues with his family and I just wanted to be there for him instead of get into a fight. He basically said that they talk about nothing and she always calls him to talk about her day. My whole thing is, he knows how I felt about the situation when he allowed this girl to contact me and harass me 5 months ago, and he made me think I was the only one he was talking to but the whole time, he’s still been in contact with the same girl and I honestly don’t know how to feel. I feel like I shouldn’t even be worrying about this because there’s a lot of other stuff going on in our lives but I hate that he lied to me