Well, sounds like your counselors are full of it, to begin with. I wouldn't take any wisdom on whether to live polyamorously or monogamously from someone who doesn't accept the validity of both options for some people.
I didn't know if I was poly or not when I started dating my partner -- who had been identifying as poly for years before we met. I knew that it fit with a lot of ideas and impulses I've had in my life, and that I felt it was worth giving a shot. Now I identify as "polyamorous" not just within my relationship but as an individual... having lived on the poly side, I would have a very hard time being in a monogamous relationship. So for me, it took some time to find out how I identified. I suspect that's true for a lot of people here who identify strongly as "poly" or "mono." They've tried both and know which one works best for them. (And there are a lot of people who could go either way, depending on the relationships they find themselves in.)
Whether it's natural for you to be in love with more than one person at a time (and my personal belief is that it's natural for most people, though not everybody) is one question; whether in your specific situation the risks of changing your marriage's monogamous structure are worth taking is another. From my perspective the second question is more important -- the first one will answer itself in time. I can't answer that for you, but I can say this: nobody's relationship is perfectly stable and healthy, not before trying polyamory, not during the opening-up process, not afterward. Polyamory will almost certainly bring out problems in your relationship you didn't know you had. I take that as one of its benefits, since it creates an opportunity to grow in ways you might not have done otherwise. The pitfall to avoid is using a new relationship to escape or evade the problems of the old one.