Why is it when my boyfriend was talking to his ex I grew more insecure and jealous of his ex, along with just rethinking our relationship?

Anonymous
Communication stopped and is done with his ex, but I'm still trying to get over things and process them.
She was telling me shit about how they were really close and she felt like he was the only one who understands him. She kept bugging me to talk to him bringing that up. Which made me feel pitty for her cause she went through a lot and I felt bad saying no. But was uncomfortable with it. I found our right away he told her something personal, he explained to me why. But it just made my blood run cold when she texted me asking to give him a hug.
I became more insecure and at times felt this sense of competition when texting her.
I started to think he wanted her too and at a point when she for some reason opened up right away about how things fell apart between them ( but didn't tell my boyfriend why things ended) when she first texted me. It made me feel like they should be together and it made me more insecure about my relationship with my boyfriend.
I found out he is codependent on her and so is she. He's defended her and it made me more insecure with myself and about us. I just felt like he emotionally cheated on me.
At a point we argued about her and he defended her and stuck up for her, by the end I felt like an asshole and I just told him he can. I went through his phone finding out he sent her a text about us talking about her and apologized trying to make sure she was okay and make her happy.
I wanted to know others thoughts. He feels deeply remorseful about it all and we have her blocked and done with her.
Why is it when my boyfriend was talking to his ex I grew more insecure and jealous of his ex, along with just rethinking our relationship?
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