I am so with you on this one. I felt nearly the same when I realized all this for the first time. Yes, it is frightening and more than any other aspect that came to my mind, I was frightened of the possible changes that could come if I act on my feelings.
And I have to tell you, at least from my personal experiences, this won't go away so easily. I fell hard for a good friend, tried to shut away everything - for 3 years I managed but at last, it was overwhelming and impossible to ignore.
I am married and I was scared like hell to tell my husband. I imagined him freaking out, leaving me, being an emotional wreck and what not, because I knew I could have been like that in a case vice versa. But some words of encouragement: he was awesome. He loved me more than his dream of a mono-family because this family would have been nothing without me. And he was so quick to process to this whole idea that I was and still am scared of his speed.
We (all three) are really new to the whole idea. We are going day by day, moving slowly, talking a lot and we will see where this is leading us. The questions Viable Alternative gave you are really good ones, I took them into consideration as well when I made up my mind. I asked myself if I could be happy with disregarding this aspect of me, with living on while lingering feelings for another person occupy my mind. If I could be mono for my husband. I couldn't. The three years proved to me that I would never be able to forget/not feel the way I did.
If that is the case for you, if you can't imagine yourself without this feelings now or in the future, than sit down together with your boyfriend and talk about it. Be gentle, try to explain how and what you feel. That is a really hard one, because even if he reacts positive, he may feel kind of insufficient and insecure. Not being enough is a point that comes up in the stories told on this forum quite often. Try to give him certainty that you have no intention to leave, that you love him, just like you did in the situation you mentioned in your first post.
I can say that for us all this mess had really beneficial effects. We bonded quite strongly because of it, we decided to open our little family for another wonderful person and our commitment was strengthened. Good luck with your decision and the upcoming days, try to find your way, it seems to me there are too many to predict how your journey will develop beforehand.
(I am still new to this, but I could relate to your feelings and had the urge to answer, I hope this can help in any way.)