How do you get over/react to finding out your SO was still seeing/having sex with another during the early weeks of your relationship?

I’ve been with my current g/f for around 9 months now, or at least our initial date was in early January. We’re both in our 30’s, looking to settle down, etc. We met through a dating site and hit it off pretty quickly.

When I first tried to make it 100% “official” (probably a month or maybe 5 weeks into it), she informed me she had still been going on other dates, and she needed a day or two to think about it. She ended up coming back and saying yes and we’re happily together now.

BUT, over the months that followed, I discovered that the other dates, had all been with one guy. One guy whom she had started seeing some time before me and dating/hooking up with for some time (at least a month). This put me off quite a bit, as my own dating etiquette has always been that if you are SERIOUS about dating someone then you are only dating them, even if you haven’t made it official yet. I would only consider dating multiple people at once if I wasn’t serious about any of them. And I’ve had this reinforced over my life experiences in dating and from friends. To that end, I was in fact dating another girl I had met online a week or so prior, and it still had potential to be anything from a solid hookup partner to a long term relationship… But I decided that I was far more into the current girl and I ended it with the other girl.

Anyways she had been seeing the guy, and still sleeping with him, whilst simultaneously seeing and sleeping with me. This really hurt me as some of our early dates were, or so I thought, really great and even super romantic and passionate. To the point where I couldn’t even have fathomed going to kiss, let alone sleep with, someone else… this putting me off as she apparently wasn’t as blown away as I was….. Then come to find later, she was even seeing us one day after another! At least twice she saw him on a Friday and then me on a Saturday (or vice versa), having a date/sex/sleeping over with both of us.

Had I found out about this early on in our relationship, I almost certainly would have ended it on the spot. As it is, it’s been an argument a couple of times and a lot of hurt feelings for me. I seriously dwell on it a lot. Her argument is that we were not “official,” and we hadn’t had “the talk” that signified we were exclusive. She has said she’s sorry that I’m hurt by this whole thing, but that in her experience it’s been regular etiquette that until you have that talk anything is game. She apparently hadn’t decided which of us she had wanted to make it official with, and had in fact been on course to do so with the other guy until I came along and won her over. This of course does nothing to assuage my feelings.

I’ve fallen in love with this girl, and I see a really great chance at a future with her. But how do I get over this recurring feeling that I’ve been cheated on, or at least fooled or shamboozled or whatever you want to call it early on in our relationship? If moments that were really awesome and meant a lot to me weren’t enough to keep her from having sex with someone the next day, then what about future moments? I’m not even sure what day to say is our “anniversary” .. I refuse to list the day we went on our first date (as seems to be the MO) since she was still seeing another guy. How do I get over this?