Mabinogi.

Not many people have played it I'm sure, but imagine this:
You've just downloaded a new free MMO. You figure it's gonna be super pay to win, but it's free so why not give it a shot anyway.
For the first few minutes, after you stop being confused by the UI, you start to take everything in. There are no classes, you can do whatever you want. Want to be a mage AND a warrior? Totally doable. Want to be a bard playing in the town square for tips? Thanks to the robust music system, you can. In fact, you're having trouble finding anything you can't do.
A few months later, things are progressing nicely. You've mastered every skill, played thousands of songs by now, got some pretty good gear, and you haven't encountered even a hint of the p2w you expected. Life is great. However, you're going to need a bit of a gear upgrade before tackling this next dungeon. You check how much it'll cost you. 300 million.
You've never even seen more than 50 million in one place before. Nevertheless, you figure with hard work, you can achieve it. After a month, you've gathered about 100 mil by exploiting market bubbles to sell anything valuable as fast as possible and in as large of quantities as possible. It's still not enough though. The cash shop begins to beckon you. You could pay a little real money to buy a cash shop item, and sell it for gold.
But you realize that in order to get the 200 mil you need, you'd need to spend over 100 dollars. You rationalize to yourself that hey, the p2w isn't that bad if it's easier to make the gold in game than it is to make the real money to buy it. You continue on your quest, but you run into an issue. There just aren't any more bubbles to exploit. You've crashed the market in your quest to obtain all the gold you need without spending a penny. You cave, and buy just a couple cash shop items to sell and make up the difference. You get your shiny new equipment. You feel powerful. It's such a huge upgrade it's almost ridiculous. You feel like 20$ was worth it to have this much fun. Out of curiosity, you check to see how much your next upgrade will cost.

2 billion. It's too late. You're addicted. Sunk cost fallacy has kicked in. You've already invested in your character, and that next upgrade is gonna cost you 2000$.
You can't quit. You've tried. There's just no game like this anywhere else. You will spend that money eventually, no matter how hard you try to avoid it.

This is my story. I'm aiming to get that gold without spending a penny. It's been months. I'm half a percent if the way there. It's not gonna happen. Every day I have to pull myself away from that cash shop. It would be so easy, but so irresponsible.
But one day I will spend that money. The game is insidious like that. The only way to avoid it is to either not play the game in the first place or not give a shit about progressing. I am in neither camp.

Genuinely, I love the game, but every day I pray it gets shut down before I have the chance to pay in that much money. It's so hard to stop myself.