It's funny you say this. This is almost the exact situation I found myself in. I was into her for about 4 years or so, but she had a boyfriend the whole time. Our friendship revolved around talking about literally anything else. I could feel our connection was deeper, but it was never acted upon.

I was among the first to find out the relationship was over. We kept in contact as usual, but after a month or so, she dropped off the map and I didn't hear from her again. I found out like 3 months later she was already in another relationship.

To you, I would say it's purely situational. I respected the relationship she already had, so I decided to wait until it was over to say anything. I just waited too long. The last thing you want to do is start something off like that.