Part of how I first got into poly was being exactly in your shoes-- thinking I was mono (though not good at it, I'll admit) and hetero and winding up falling for another couple... Yeah. I remember those days.
It's okay to not be sure about your sexual preference. Many, many people at every age change the label they've been using because they realize it doesn't fit them anymore. It's also okay these days to not really have a label-- so many people feel stuck between preferences and genders, and have been coming up with their own words to describe themselves (pansexual, heteroflexible, etc.). In addition to my (male) life partners, I have a girlfriend (who has her own boyfriends). I tend to label myself as bi simply because it fits the facts, and I try not to over-analyze that, but she considers herself straight.. and we still have a relatively happy, healthy secondary relationship.
I want you to know that what you have sounds beautiful. The biggest tool you have, that anyone has, for staying on the same page and keeping any relationship going is hardcore communication. It's technically true of both mono and poly relationships, but it's easier for just two people to get by on less explicit strategies than it is as numbers grow.
It's great that you've gotten to the point with your girlfriend where you can say "I love you" to each other comfortably, and to me that's a clue that things are pretty stable. However, you should definitely talk with all three of your partners-- maybe your husband or your girlfriend first, if you want to broach the waters before a group conversation. Tell them that you're feeling increasingly invested in the relationship and worried that the sentiment might not be mutual. Ask them how they're feeling. Find out what everyone imagines this relationship looking like in a few more years.
How old are your kids? There's lots of threads around the forum on what to say and how and when. If they're younger and you're comfortable telling them a great deal, I recommend the book I Love You the Purplest
by Barbara Joose. It's actually about sibling rivalry for a parent's affections, but it conveys the idea that you can love two people a great deal without having to love one more than the other. (The mother loves one son the "bluest" and one the "reddest.")
One more fun secret... Every single aspect of your problem can be approached from a research angle. Read books, google terms that catch your interest, and keep coming back here. Knowledge is power.
Thanks for sharing! We're here for you.