Hi there - thanks for telling your story - you asked for comments - here goes.
Twice you mentioned the word "accidentally" when you talk about getting into the relationship with the other couple. I need to sort of take you to task with this, because it was far from accidental. You made conscious decisions to take certain actions and to so certain things. Those are not accidents. it may not have been something that you originally planned, but calling it an "accident" can sound a little like not taking responsibility for what has been going on, and I really think that you need to own it.
How much trust do you want your husband to have for you? For me (and others may differ) having a solid trust in my relationships is the keystone of them - without trust, the whole thing falls down very quickly, or you expend a ton of energy just to keep it up. Sharing how you feel with your husband, and dealing with that should be part of owning things, and taking responsibility. You say you don't want things to change, but right now you are in a position where you sound uncomfortable not telling your husband everything, so it sounds like you DO what some things to change.
I am a big believer in telling the truth to those we have made a commitment to and working through issues that come up as a consequence.
Oh and when it comes to terminology, you are in what is called a "W" - you are one of the "hinges", since you have two relationships.