Another brother bites the dust, can we get an F in the chats please

Obligatory "this happened two days ago."

I had been together with my girlfriend for almost 4 years. I proposed to her about a month ago, she said yes, and we were incredibly happy! Or so I thought.

About 3 weeks after our proposal, I notice she starts acting different. Not having sex with me, looking away/down when we kiss. She had also been spending way too much time (at least 4 days of M-F each week) with her coworkers drinking beer after work, driving home drunk, and often pretty late. I went to a few of these gatherings but didn't really enjoy getting slammed on a monday night when I have work the next day, so I often opted out.

We would also share our google location with each other at all times. Mostly because she traveled to sketchy places for work, and I would regularly go on trails, so it helped each of us know the other was safe. I looked at her location one day and it was turned off. I texted her about it and she said that she kept receiving notifications about it so she had turned it off. Hadn't been a problem for the past year. She turned it back on, and it was off again the next day. She also had two phones, a personal and a work phone, which she would keep both with her at all times. I only had the location for the personal phone. Eventually she stopped using her personal phone and only used the work phone.

Two days ago, about a month after the proposal, I decided to snoop because my suspicion was at it's highest, and I just wanted to put it to rest. I used her computer to log into her gmail account and looked at her timeline. She had been going to an address across town about 10 times in the past two weeks. I called her immediately because she said she was at the office finishing up some work that was due the next day. She said she was at work, but leaving to go to the grocery store, which she then did.

I waited until she got home and confronted her about it. She said she had been meeting a guy from work just to talk and hang out, but she didn't want to tell me because she thought I would get mad. Turns out, she met him for the first time about 2 days after the proposal, and started seeing him at his house within that week. I kept prying, asking her more questions, she told me they had only kissed twice, then it turned into they had made out, and she denied every accusation of sleeping with him every time I asked.

The next day when I got home from work, I asked to see her phone to read the messages between the two of them. She said she deleted them. So I said I wanted to look at it anyways, just to be sure. I started recovering the data from the last backup. She sat down beside me while I was doing it, and asked what I was doing. I told her, and the look of panic in her face was real! So she starts talking about feelings and all this other crap while the phone is recovering. I asked her one last time, "before I look at this, is there anything you want to tell me?" She was silent. I asked flat out "did you sleep with him?" and she said yes. Turns out that she met with him almost every day, starting about 8 days after the proposal and had sex with him most of those. Funny enough, the data recovery didn't even work. So that's a win I guess.

Planning on moving my stuff out later this week to a new place. Havn't told her yet. Still can't believe she started cheating on me less than two weeks after she said yes to my proposal, and even more so with someone she met a few days after the proposal. Feels shitty but I'm keeping my head up.

TLDR; TIFU by looking at my fiances location history and found out she started sleeping with another guy less than two weeks after she said yes to my proposal. It has been one month since the proposal now.

Edit: Wow! Thank you all for the encouraging comments! They help so much more than I would've thought. Some of these really got to me, and reading that I should've posted this as a lifeprotip made me laugh. Most if all, it helped me confirm that it was not a FU on my part, but actually a gift that it happened so early. Thank you all, so much! Going to read through and respond to as many as I can.

Edit 2: I know that this post was meant to be posted in another sub, but thank you for being kind, and not focusing in on that aspect haha