'I was totally cool with my boyfriend's girl best friend, until I found an old photo.'

Dating a guy whose best friend is a girl is not for the weak. When Matt* first told me about Anna*, I got an embarrassingly huge case of the "pick-mes". I acted like I was sooo cool with it, while internally every insecurity painted a picture of what this girl – and their relationship – looked like.

But, from the start, Matt and Anna were the epitome of platonic friends. They had that rare kind of connection where you just knew there was nothing romantic between them. Their friendship was so openly, confidently platonic that - once I saw it - I never once felt insecure or jealous about their bond.

Anna herself made it easy to feel that way; she was cool, laid-back, and effortlessly kind. She didn't hover around us or cross boundaries. It was the kind of friendship that seemed so rock-solid and transparent that I couldn't help but feel grateful to have it in my life, too.

At first, some of my friends and family were a bit suss about Matt's friendship with Anna. I could see it in the raised eyebrows or the sideways glances whenever I mentioned her. A few friends even tried to warn me, saying they'd never be comfortable with their partner being so close to a girl, but I brushed it off. Over time, though, they came around. They saw how easy and natural everything felt when Anna was around, how comfortable and lighthearted our dynamic was when the three of us were together, which we often were.

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It honestly never felt like a third-wheel situation; it was just the three of us, and I actually loved it that way. Anna brought this infectious, wild energy to our lives. She was the first one to suggest spontaneous beach trips, always had the best restaurant recommendations, and could talk me into seeing even the most obscure bands at last-minute gigs.

Anna was there as much for me as she was for Matt, and I thought we'd struck a kind of relationship jackpot. In fact, when Matt and I scored tickets to anything, it was just a given we'd grab three. Every festival we went to, Anna was right there with us, blending in so naturally that I didn't think twice about it.

Sure, Matt and I would do plenty of things on our own, but Anna was always woven into the fabric of our biggest moments. I'd become grateful for her. She made life fun, unpredictable, and even in the middle of it all, I never felt like I had to compete for Matt's attention. I just assumed she'd always been the 'fun friend' - the kind of person he was lucky to have around.

When Matt and I moved in together, I was sorting through my stuff in the spare room. We'd converted it into a kind of shared space office that we could both use when we had work-from-home days. Matt had already claimed his half, and it was a mess. The shoe boxes of crap he'd stuffed on shelves were not the aesthetic I was going for, so I began to sort through it with the intention of heading to the shops to grab him some baskets to make it look a little better.

It was mostly crap – receipts, old childhood sports trophies and a few broken pens. But what did stand out were some Kmart photo envelopes. We're digital-age kids, we don't print photos, so I assumed it was childhood memories, maybe a few embarrassing photos I could tease him about. But when I opened the first one, my stomach did a little lurch.

There, staring up at me from the glossy photos, was Matt. And Anna. Not as friends, not as festival buddies - but as a couple. There they were, kissing on Narooma beach – a place we all loved to go to together to escape the city. A young, lovestruck Anna leaning against Matt, his arm wrapped around her, grinning like they'd just shared the most private joke.

I stood there, frozen, trying to process it. How could they have been a couple? All this time, and neither of them had said a word. Suddenly, every memory of us three together felt tainted, like I'd been cast in a play without knowing the full script.

I couldn't hold it in, I typed them both paragraphs and then deleted them. I couldn't call them, I couldn't call anyone, I was a mess.

When Matt got home he sensed the tension immediately, trying to gather me in for a hug and a neck kiss asking what the matter was. I handed him the photo envelope as casually as I could manage. "Mind telling me about this?" I asked, trying to keep my voice from shaking.

Matt pulled out the pictures and his eyes widened. "Oh, uh… yeah, these are… old," he said, fumbling for words.

"I f**king hope they are old!" I yelled. "Why didn't you tell me you and Anna used to be together?"

He looked down, then back up, and then the truth spilled out.

"Yes," he finally admitted, exhaling like he'd just crossed a finish line. "Anna and I were together for a few months. But it was ages ago, Kate*. We knew we were better as friends and I didn't think it mattered."

Didn't think it mattered. Right.

He went on to explain that after they broke up, they'd slid back into a friendship like it had never happened. "And by the time I met you, well, I just didn't see the point in bringing it up. It was over," he added.

I tried to keep my cool, but I couldn't help but feel blindsided. "Did Anna know you weren't going to tell me?"

Matt hesitated just a little too long, confirming my worst fears. Of course, she knew. They'd kept their little secret, sharing a bond that I'd never been fully aware of, and I'd been the clueless partner along for the ride.

When I thought of Anna, I was filled with the kind of rage that can only be born from hurt. Here was this person I'd trusted, laughed with, formed a friendship with, and all the while she'd been keeping something from me. All those years of festival fun and inside jokes suddenly felt loaded. How much of it was about me, and how much was just the two of them, reliving some private world they'd built together?

I'm not proud of it, but I rewrote those (slightly unhinged) paragraphs and sent them to Anna, telling her never to speak to me again. My other friends rallied, supporting me and trying their best to keep their "I told you so" to themselves.

Anna stayed away, she was hurt too, but she was respectful enough to tell Matt that she wouldn't even speak to him while we tried to sort out our issues. She just wanted everything back to how it was and would do anything to get it there. I'm grateful that she didn't kick back or try to make it worse.

Matt, for his part, kept brushing it off like it was no big deal, telling me it was in the past, and that it had ended long before I came into the picture. And maybe he was right. Maybe I was making too much out of something that no longer mattered. But as much as I tried, I couldn't shake the feeling that I'd been left out of something. I wish they'd just told me from the start.

We got through it, eventually, and I try to focus on the memories we were making now, the ones without Anna in every single one.

We see Anna sometimes, but nothing like before. I miss how we were, and I do actively try to make an effort not to feel so weird about her, but in those quiet moments, the little voice still creeps in. The voice that tells me they'd been each other's person first and they'd kept that from me. No matter how I try, that feeling never quite goes away.

*Names have been changed due to privacy.

The author of this story is known to Mamamia but remained anonymous for privacy purposes.

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