How do I tell my best friend that I’m dating one of his longtime friends?

Hi everyone, I’m looking for an outside perspective on how to handle a really delicate situation that’s been making me feel terrible.

A former college classmate of mine (let’s call him Brian) and I have been friends for about nine years. We’ve traveled together, we talk basically every day—even if it’s just a text—and there have been times when, despite work and other commitments, we still managed to meet up on weekends to hang out. Lately, for many reasons, our in-person meetings have become much rarer.

Brian is very unique in the way he interacts with others, especially with women. He has always seemed uncomfortable taking about relationships, even if they concerned other people. I’ve never heard him talk about crushes, relationships, him seeing someone and so on. When we first met, I wondered if he could be interested in me—not so much because of his behavior, more because we were two friends of the opposite sex. At the time, I was in a relationship; a few months later, I broke up with my boyfriend, then I was single for a couple of years, had another relationship that lasted about a year, and finally became single again.

I’m a pretty private person when it comes to these aspects of my life, so I never told him the details of my relationships, but I’ve always been upfront about whether or not I was seeing someone. He, regardless if I was single or not, never made a move or hinted at anything. Over time, I had therefore completely ruled out the idea that he might be interested in me. On my part, I never felt any romantic attraction toward him.

At Brian’s graduation, about five or six years ago, I met Josh. He’s one of Brian’s best friends from high school: Brian had often told me about him, and apparently Josh had heard a lot about me, but we’d never met until then.

Unfortunately, as far as I was concerned, I liked him straight away, from our very first conversation. The problem was that he’d already been in a relationship for a couple of years. For that reason—and also to protect myself—we only talked every now and then and mostly hung out as a group of three: me, Brian, and Josh. I did everything I could to keep my distance, because I realized that the more I talked to Josh, the more I liked him, and the fact that he was in a relationship made me miserable. I was convinced of two things: first, that sooner or later I’d get over it (spoiler: it didn’t happen); second, that he and his girlfriend would eventually get married and start a family, because they really seemed like a very solid couple.

Then two things happened that completely turned this situation on its head.
The first was that, three years ago, while I was living away from home in a city far from Brian, he sent me a letter in which he confessed his feelings for me. I replied honestly, telling him that I didn’t feel the same way and that, if he needed to distance himself from me to heal, I would understand, even though it would break my heart to lose his friendship. He replied that we would get through it together.
For a while, there was inevitably some awkwardness, especially on my part, but over time we returned to a dynamic that felt very similar to what it had been before. More or less. Since then, we’ve never brought up the subject of his feelings again.
The second thing is that, about a year ago, Josh broke up with his long-term girlfriend. Six months later (so in January of this year), he kissed me. From there, we started dating.

I didn’t want to tell Brian right away because I didn’t want to put him in that situation if things between Josh and me were just a fling. I preferred to wait and see if it was really something serious. Now that I can say with confidence that it is, I feel the time has come to talk to Brian about it.
Actually, Josh had already offered to tell him himself at the beginning of our relationship. In fact, keeping this “secret” weighed more heavily on him than on me. Just the idea of it, though, caused me tremendous anxiety, so I’d asked him to wait.

What do you think is the best way to handle this situation? Should Josh be the one to tell him first? Or should I be the one to talk to him? And if so, how would you handle it if you were in my shoes?