TIFU and made assumptions that my girlfriend was on a date before actually finding out the whole situation.

Today I royally fucked up.

So, I went to work where I begin at 3pm and work until 1130pm. The whole time my girlfriend is texting me that she’s lonely and depressed. I felt bad because she let me borrow her car therefore leaving her stuck at home. I work in a call center so its hard to have a convo with her. Anyways, I leave for home and stop by to get her flowers in hopes to cheer her up. I get home and she's not there. I look all over the house thinking she's hiding and trying to scare me. So after my search I finally decide to give her a ring. I call her and my phone drops the call. A minute later she texts me and says that she's with her aunt. I leave it at that. I know she was feeling lonely and depressed so I’m happy that she went out. While I’m at home I decide to cheer her up and clean the house. Top to bottom. Clean the living room, the kitchen, the dishes, the bathroom, bedroom, vacuum, etc. While I’m wrapping up my sister calls me asking what I’m doing. I say nothing just hanging out at home. She then asks me why my GF is downtown with another guy. Immediately I see red. I jump in the car and head down there quick. While driving I call her with no luck, call her aunt (she's really close with her aunt so she would know if something was going on) and it rings then goes to voicemail. I head into the bar to find my girlfriend talking to this guy and I say thats my girlfriend, as I push him aside to get to her. We go outside and she's furious. I’m having a rage blackout right now just yelling at her while she's yelling right back. She tells me that she was out with her aunt and uncle with these people and her uncle got sick and they still wanted to go out so they did, my girlfriend, the guys and five other people. While I’m trying to get her to see my side of the story she says that I’m psycho for showing up and says were done. I leave the bar alone. As I’m leaving I see the group that she's with, something that my sister who has had a vendetta for my (former) girlfriend failed to mention.

While I’m back at home her aunt calls and confirms and explains the whole story that they went out, her uncle got sick and had to go home. She gets home and explains that we are in fact done because I can’t trust her at all. Three year relationship, living together, getting ready to take a trip to the west coast where I was going to propose in the next two months, done.

TL;DR I fuck up and made assumptions that my girlfriend was on a date before actually finding out the whole situation and embarrassed my girlfriend amongst her friends.