Fiancée wanted to explore other men, then came back pregnant. But I had already moved on.

I am writing this at 3:00 a.m. because I cannot sleep, and honestly, I need to get this out of my head before it drives me completely insane.
Eight months ago, the woman I thought I was going to marry told me she needed to "experience life" before we got married. Tonight, she showed up at my door pregnant and desperate, expecting me to welcome her back like nothing happened.
The problem is, I am not the same guy she left behind.
Let me back up.
I am Dex, 30. Been working as a diesel mechanic at a trucking company for 6 years. Good job, steady money. $72K a year, which is not bad around here.
I was engaged to Nia for 3 years, together for five total. We had everything planned out. Wedding next spring, house down payment saved up, the whole domestic dream.
Then one night last March, Nia sits me down and drops this bomb.
"I need to experience other people before we get married."
I am sitting there like someone just spoke to me in a foreign language. "What do you mean, experience other people?"
"I have only had serious relationships," she said. "I have never just dated around, never explored what else is out there. I need to know what I am choosing when I choose you."
The way she said it made it sound so reasonable. Like this was just good sense. Like I should be grateful she was being so thoughtful about our future.
She wanted 6 months. 6 months to get it out of her system, and then come back ready to fully commit.
"I love you," she said. "But I need to know I am settling down with you because you are what I want, not just because you are what I am used to."
I should have told her to go f*** herself right then and there.
Instead, I said okay.
Actually said okay to my fiancée asking for permission to sleep with other men while I waited around like some kind of loyal dog.
She moved out the next week. Took her stuff, kissed me goodbye like she was going on a business trip, and said she would see me in 6 months.
For the first month, I was a wreck. Could not eat, could not focus at work. Kept thinking this was all some nightmare I'd wake up from.
My buddy Ryker at the shop kept telling me I was being an idiot. That no woman who really loved me would need to explore other options first.