I would second Arrowbound, if you told her and the thing is up in the open, this will be a problem when telling her now that you want to be with other people, trust has been broken once, why not twice could be her reasoning. IF you didn't tell her back then, she may have guessed something, you have to be prepared that this stuff will come up again and then it will be ugly if you now want to tell her that you want other relationships. You have been dishonest with her already, why should she believe you that you will behave differently this time?
Aside from that problem, I know what it feels like to wait for years to finally stand up for your feelings and the person you have become. I have been in a similar situation and had to confront my husband with a new love that had developed over time already. The worrying about what might be and the what-ifs had been harder than the actual conversation. There had just been a point when I couldn't take to ignore myself anymore and I choose me over the status quo that we had established. When I did it, I noticed that even if this was a big step, my husband and I had done those steps all the time. It was another adjustment we did to the changes the respective other had gone through over the years. Each change could have been the end of the relationship if the other would have said 'That's it, I have had enough.' but we never did. It was a proof for the health of our relationship and each other's wish to stay together.
Wishing you luck.
Facts: 30, female, bi, v-type relationship with Sward (husband, straight, mono) and Lin (boyfriend, straight, mono), poly-fi and co-primary.