Cheating In My BF With His Cousin

I have been with my boyfriend (31 M) for 2.5 years now but have known him most of my life because we were childhood best friends.

I love him. I really do. I am happy with him. Our sex life is great. Better than great.

Which is why I am so mixed up about why the hell I am cheating on him with his married cousin and why can't I stop?

His cousin (28 M) and I have worked together for almost 10 years now. Heavy machinery mechanics and maintenance. Up until recently I would of never thought he was anything other than straight. He is married to a woman.

I honestly don't even really like the cousin. He's a skinny little loud mouth. Big shit talker. That guy on jobsites that just won't shut the hell up. Doing everything but his job. But his dad owns the business so what can you do.

So this cousin obviously knows I'm not straight and has often in the past teased or harassed me about it while at work. I can usually ignore him. But one day on a job it just me and him and he's just not dropping it. Making all kinds of gay jokes, being really crude.
Not sure where my head was at but I end up making some kind of comment about if he he's so damn interested that if he doesn't shut up I will shut him up with my cock down his throat.
(Good spot to point out i am taller and more muscular than him).
Well shit if the little brat doesn't call my bluff and challenge me to it. Laughing. I was so thrown off at first but also so irritated with him I figured I would scare him a bit and meet him in this ridiculous game of chicken he was playing.
Ended up going after him, forcing him to his knees, grabbing the back of his head and grinding my crotch in his face.
I figured that would be it, that he'd back down and want out.
But then the little shit makes come kind of crack about me probably having a small dick.
I absolutely do not.
Honestly pretty proud of my decent sized, thick, uncut cock.
So... I unzipped.
And stuffed my half hard and growing cock in his mouth.
When he didn't fight me, bite me, or push me off I knew he wanted it.
But I wasn't going to give it to him easy.
So I straight up FUCKED his face as hard as I could. No mercy. Both hands on the back of his head and bucking hard. My cock pounding the back of his throat.
He was gagging and making choking noises and I was loving it. I knew it had to hurt. I wasn't being gentle. I was so sick of him. So sick of his shit. I wanted to hurt him.
I don't remember all I said to him but I know I called him every degrading name I could think of as I throat fucked him there on the dirty floor of some industrial shop.

I had been making sure he was getting a little air but when I got close to cumming I decided I was going to choke the little slut. I pushed in all the way, deep and held his head as I came down his throat.
He took it pretty well but towards the last few seconds he struggled against me. Needing air. His face red and tears streaming from his eyes. I held him a few more beats before letting him lurch backwards coughing and gagging.

I felt so , I don't know powerful? Dominant? Victorious?

I threw a few more insults at him before zipping up and returning back to what I had been doing.

He disappeared for a while before coming back and getting to his work quietly.

I thought that was going to be the end of it. A one time thing. I had put him in his place. It hadn't even really felt like sex. It felt like violence. The same as having beat him up. Right? So I justified it to myself that it wasn't cheating. Not really.

Well.... I was wrong.

Now the little brat knew just what to say to me to get me riled up. Knew just how far he needed to push me to make me break. Knows just how to get what he wants.

Since that first time I have face fucked him hard like that 4 times and violently fucked his ass 3 times while at work.

I was honestly getting such a high off it. Feeling so big and on top.

But the more it keeps happening I think the rush is wearing off and I can't lie to myself anymore. I'm cheating. I'm cheating on my best friend.

What the fuck am I doing?
I want to stop. I want to go back in time and undo it all.
But I don't think I can stop.
Apart from quitting my job and moving across the country....