First of all, let me give you all some context.
I'm dating a guy and our anniversary is two weeks from now, he's very sweet and always is there for me when I need him, our relationship is very healthy. He's also been my only company over these last months, since all my friends forgot about me during vacation (I'm from Brazil so we get a two month holiday break at the end of the year).
The other person involved is actually my best friend but because of some coincidences, misinterpretations and a lot of anger we stopped talking for almost an year and she spread some nasty rumors about me, so we actually hated each other for a long while. 
At the time she was dating my bf's best friend, but it was an unstable and toxic relationship for both parts, he's also one of the motives we argued. 
My boyfriend, started hating her because of all the shit she put me through and for dumping his best friend. 

Three days ago she asked me to go out and talk about all of that. Before accepting it, I talked to my bf. 
I never expected him to be so mad at me for wanting to talk to her. For real. We argued for like, two days. I accepted her offer anyway, but he was still pretty enraged. He didn't think she deserved it, that I am pretty dumb for doing that, and that I am gonna be betrayed by her once again.

Today, he asked me if he could come over so we can watch some movies together, I sweetly declined because I had to go talk to the ex-enemy/best friend (I guess), and said that if we finished it early then I would go and meet him. 

Well, that's where things truly go downhill. 

I went and talked to her, we had a good conversation about everything that happened, she apologized, recognized her error and didn't blame me for anything. At that point I had pretty much accepted her apology and we were talking about random things that happened over the course of the last year.
That's when my bf called me and said he had been waiting for over two hours for me to go meet him and started saying that I preferred her over him. He told me he was gonna meet me at where we were and that if I didn't stop talking to her, he was gonna break up with me. 

I got really mad and told her everything, but he wouldn't stop calling. He was almost screaming telling me to go meet him at the park and that he didn't approve of me to be friends with her again. I, very emotional at that point, told him to come where I was if he wanted to talk, I told him he was being very childish and that it was only a conversation. 

When he got there, he wasn't screaming anymore, instead he very calmingly asked me to speak in private for some minutes. But I was very angry and told him I didn't want to talk to him.
We started loudly arguing in public and I was ready to dump him. He told me I was being a jerk and "if you really do prefer her, then I'm not gonna be with you", so I replied giving him my promise ring. I was in tears at this point, I had completely lost it. He was pretty shocked and my friend was quietly watching everything. 
I grabbed her hand and walked away from him and towards the park (we live on a really small town). He followed us, of course, while rambling about giving him one minute to talk to him in private and not breaking up.
Me and her sat down at a bench and he was still going on. I was crying a lot and asking him to go home. I told him I was gonna talk to him tomorrow but he was still telling me we shouldn't leave this unfinished.

I did my best to calm myself down, he gave me my ring back, and we talked for some minutes.
He apologized to me and told me I really shouldn't be friends with her because she hurt me, and that he was only worried about me, but still very angry. I told him that it was my choice and he had to respect that, and that it wouldn't affect our relationship, that I would still give him attention. We hugged and I thought everything was okay.

Nope.

She walked me back home and I apologized for the trouble, she said no problem and went home.
I went to my room and grabbed my phone.
I read some recently sent messages from my bf and he still insisted me to choose one of them. I of course said that I could be his gf and still be her friend, and then he said he didn't want me anywhere near her. I asked him to come over so we could talk. AGAIN.

We talked, he told me how embarrassed he got at the ice-cream shop and that everyone was gonna talk about it tomorrow. He also expressed how upset he was that I lied multiple times about seeing him and not becoming friends with her. I never said I was going to meet him, just said it was a possibility.

With no option left, I apologized about my behavior and told him that I am gonna stop talking to her. He's still upset and we can't find a common ground.
I still love him very much but I miss my best friend friend.

Tell me reddit, who's guilty? 