You can word your confession with a qualifying preface.

Based on what you wrote in the above post, you would not confess your love to woman who had a bf. Say that to her.

However, and here comes the qualifying part, you hadnot known that she was in a long distance relationship until 2 years into your friendship with her (I might be a little off with dates, but you know the timeline best).

Without realizing that you might step onto somebody else's turf, during that long period of friendship, you allowed yourself to develop feelings for her.

You are certain that you would have nipped those feelings in the bud had you known from the start that she was in a relationship.

In no way do you blame her for not mentioning her relationship status sooner.

Rather, you are simply explaining the context and providing a backstory.

Now, having felt so deeply for her for so long, you have reached a point of no return.

And at this juncture, without imposing on her or trying to win her over, you simply want to confess your feelings for her and express hope that she would allow your friendship to continue as before despite your confession, and that said confession would not make her uncomfortable. If her existing relationship is to her liking, you hope she will continue with that other gentleman and you do not wish to cause her any discord.

If any of these short paragraphs ring true, you can experiment in incorporating them into how you ultimately tell her.