Breakup Advice/Rant as the Dumper

Hi y’all, I need to vent and kinda ask for advice. Warning, this is a long post.

So I've (27F) been seeing this guy (34M), let's call him Jack, for about a year and a half. He joined my existing poly relationship. At the time I was living with and dating my long-term partner of six years, Brian. I was completely upfront with Jack from the beginning, and he said he was intrigued by the idea and wanted to see where it went.

We met at work. We were both servers, and he got promoted to management pretty quickly, so we had to keep everything quiet. Honestly, I think that made it even more exciting. All the girls wanted him, and there was something really addicting about knowing he was with me.

It didn't take long to realize he had a drinking problem that he'd hidden from me. I'd ask him what he was drinking and he'd lie straight to my face. Later I found out he was finishing a 1.75L bottle of vodka every 3-4 days and would even sneak drinks in the bathroom. Looking back, that should've been the point where I walked away, but I didn't. I understood addiction and mental health, and I wanted to believe he could get better.

Outside of that, I was absolutely crazy about him. He was handsome, funny, charming, affectionate... and he love bombed the hell out of me. The chemistry between us was unlike anything I'd ever experienced. We had sex almost every time we saw each other, and I've honestly never felt that kind of connection with anyone else. It was addicting.

Everything changed when I found out I was pregnant with his baby about three months into our relationship.
I panicked. Brian was understandably upset. I ended up having an abortion for a lot of reasons I won't get into here.
Jack completely fell apart.

After that, he started talking to other women and lying to me about it (though I suspect he was doing that from the beginning he was just getting sloppy). One of them even worked with us.

I’d ask why, he held the abortion against me for the rest of our relationship. He also decided he didn't want to be poly anymore and told me he wanted me to leave Brian and be with only him. At the same time, Brian wanted me to stop seeing Jack because of everything that had happened.

I was so lost.

I honestly still don't know why I stayed. Maybe it was because I liked the excitement and the chaos. Brian is kind and stable. I grew up in a dysfunctional family, and if I'm being honest, the relationship with Jack felt familiar in a weird way. I also wanted to help him. And the chemistry between us made it so hard to walk away.

So we stayed in this weird situation for another year.
Brian wanted me to choose him. Jack wanted me to choose him. I wasn't ready to choose either of them, and everyone ended up resenting everyone.

Jack kept talking to other women behind my back though we still talked of exclusivity while we figured things out. Brian and I took a break from our relationship because I was a mess and it wasn’t fair to him.

As far as I know Jack never physically cheated, but there was constant emotional cheating, lying, hiding things, and only giving me half truths. I'd ask him if he was talking to anyone, he'd swear he wasn't, then I'd end up finding messages proving he was lying. God, the lying messed with my head so much.

I kept making excuses for Jack. I told myself he'd never had a healthy relationship. That nobody had stayed long enough to push him to be better. I encouraged therapy. I encouraged him to get sober.

Then he got a DUI.

I still stayed. I drove him everywhere. Helped him get his life back together. I was there through all of it but it still wasn't enough. He told me if I really loved him I would be with him. I asked wasn’t me being there during one of the lowest points of his life not prove that I care and love him?

When I found out he was talking to another woman, I fell apart because he lied about it. I asked again why? He said he talked to other women because of the abortion and because I wouldn't fully leave Brian. I told him I wasn't ready. I needed time to figure out what I actually wanted without either of them pressuring me. Then he broke his 7 month sobriety and started drinking again. I went to AA with him and was understanding. Then, he took it too far.

We play on the same softball team, and one night he publicly accused me of sleeping with basically the whole team because he'd stopped sleeping and had become really paranoid. He posted it on social media, threatened one man specifically. It was embarrassing and I was done.

(Over the year and a half, we had broken up and gotten back together a few times. And I'm not completely innocent either. At one point I talked to other people out of retaliation, and I know that really fucked him up even tho it was only for a day or so and I never met up with them. And it wasn’t at all sexual.)

After the softball incident, I took a month of no contact. We reconciled. Then somehow it got even worse.

He started actually meeting up with women he'd met online. He swore nothing physical happened, but said he wanted to talk to people who "didn't just argue with him" and who didn't know his past. Meanwhile he was still lying directly to my face while telling me I was the one who couldn't let things go because I kept bringing up old issues... except they weren't old issues because he was still actively doing the same things.

The final straw was finding out he'd met a girl on Feeld, lied about it multiple times, and had gone on several dates with her. This happened while we were struggling with just friendship. I wasn’t mad he went on the date. I was mad he lied.

I had remained his friend, his lover, his whatever, during all this because it felt like I was abandoning him if I actually left him for good.

Now, I’ve blocked him on mostly everything. He had initially tried emailing me to ask why I stormed off and blocked him after I found out about the girl from Feeld. I told him I wasn’t ready to be his friend and I truly needed space and that we were done.

It's only been a week and I feel like I'm losing my mind. I think about him all the time. I worry about him. I miss him. There are still moments where I just want him to hold me.
That’s when I read a list I made on my phone of every lie, every broken promise, every accusation, every time he made me question my own reality just to remind myself why I left. I know he isn't good for me. And I think I feel this intensely because of how done and how exhausted I am.

Brian and I still live together. He's my best friend. Our lives are completely intertwined, and he still wants to be with me even after all of this. But there's still this tiny voice in the back of my head that says Jack's name.

What is wrong with me? Why did I keep going back?

Has anyone else been in a relationship like this? Where you know someone isn't good for you, they've hurt you over and over again, but you still can't stop missing them? How do you finally let go when your brain knows leaving was the right choice, but you still want them back?