M26 I’m cheating on the person I want to marry, and I genuinely don’t understand what the fuck is wrong with me.

26M. I need someone to tell me what is happening to me because I genuinely don’t recognize myself anymore.
I have a girlfriend I love. I want to marry her. I have never thought of myself as a player or someone who casually jumps between women.
Yet over the last two weeks, I’ve become exactly that person.
I’ve been flirting with random girls. Talking to women on Snapchat just for attention. Seeking validation. And a few days ago, I made out with another girl while having a girlfriend I supposedly love.
The worst part?
I don’t even feel like I wanted those girls. I wanted to feel wanted.
A few days ago, I started talking to a girl on Snapchat. It was supposed to be meaningless. Then she started feeling genuine, and I started enjoying her attention.
She eventually tested me.
She messaged me from a second account while simultaneously talking to me from her real account. I didn’t know it was her. I ended the call with her real account, said I had something to do, and then called the other account.
I heard her voice.
It was her.
She had caught me.
She removed me afterward.
And somehow, losing a girl I’d known for FOUR DAYS fucked me up more than it should have.
That’s when something clicked.
I think I’m addicted to being wanted.
When someone gives me attention, I feel good. When they pull away, I feel empty. And instead of sitting with that emptiness, I find someone else to fill it.
I’ve been doing this while being in love with my girlfriend.
And now I can barely look her in the eyes.
I’ve broken rules I spent years living by. I don’t recognize this version of myself.
I’ve already cut off my ex. I’m removing the random girls. I’m done seeking validation from other women. I’m seriously considering deleting Snapchat altogether.
But I need to understand something before I destroy the relationship I actually want:
Why am I sabotaging the thing I don’t want to lose?
Is this fear of abandonment?
Is this self-sabotage?
Am I keeping other women around because some part of me is terrified that my girlfriend will eventually leave me?
Or am I just a selfish asshole who has finally stopped being able to hide it from himself?
Don’t tell me I’m a good person. Don’t comfort me.
Tell me what you think is actually wrong with me—and what the fuck I should do before I completely destroy my relationship.