My ex betrayed me then called me toxic

Before I start, I just wanna say that I don't have any feelings left for this guy. It just pisses me off whenever I recall what he did to me.

We were in a relationship for 11 months. Let's call him R. R was my first bf that lasted that long, so I was really excited for the ff month.

However, a month or two before our anniversary, there was this one guy (let's call him K) who flirted with the both of us. Back then, I wasn't open to being in a throuple. This guy found out about R because I was constantly flexing him online, and K was a long time mutual FB friend of mine.

K already had a crush on me before, but he wasn't exactly my type. One time, he went live and we were sort of friends by then so I joined. He noticed me and called out my name. His friends who were with him asked who I was and he joked that I was his crush but we couldn't be together since I was already taken. I jokingly said that he can join the relationship if he wants to (again, we were pretty close already atp). Since then, he would constantly flirt with me and my bf but I thought it wasn't a big deal and paid it no mind.

Anyway, R and I were on and off in the last 2 months of our relationship because I was constantly jealous. He had a lot of people in his dms and he would respond to all of them. And when I point it out, he'd just say, "I don't want to be rude and not respond to them." Admittedly, I may have been toxic by constantly breaking up with him.

Two weeks before our anniversary, I broke up with him again and I don't even remember the reason why anymore. I thought we'd get back together just like any other breakups we've had, especially since our anniversary was just around the corner. Guess what? On his birthday, the day before our anniversary, turns out that he was constantly calling K already. I found out about it because K posted a story of a screenshot of their call with the caption "mine". R's face was covered but I could tell it was his forehead and background. It shocked me.

A week later after our supposed anniversary, R and K were already in a relationship. I ranted about their betrayal online then he called me toxic for it. I felt betrayed by two people. Should I have just said yes to the throuple? Why did I not see this coming? I was jealous with other guys only for it to turn out that a "close" friend would betray me.

And the only reason I was even constantly jealous with other people is that because (I've never told anyone about this before) when we were in the first month of our relationship. He video called someone I knew and flashed his dick because he got horny.