Major major kudos for your handling of things. Strike a blow for honesty -- you did the hard thing and it totally worked, so cool!! I bet that he admires your bravery, he must know it can't have been easy.
As for the "am I poly" thing... I believe that you can have a solid relationship based on compatibility, companionship, and a strong partnership where the issues have been worked out together via the hard work of years. I mean, that's nothing to sneeze at! If you can make a go at having that *and* the thrill of new love and emotion and romance, with everything done with integrity and on the up and up... well heck, why not see how lucky the universe will let you be?
No way to tell what will happen... there is, indeed, no manual... but poly/mono relationships certainly *can* work. Truly, assuming a continuation of open-mindedness, honesty, amd hard work on everyone's part, anything the three of you can work out together is possible.