It sounds like you followed your heart to the best of your ability. Kudos to you in a difficult situation. It's hard to walk away from a relationship that worked on many levels but did not satisfy in a critical way.
If you want a mono relationship, then that is what you should keep seeking. For whatever reasons, your ex could not be monogamous with you. It would have been great if you could have worked towards developing a long-term V with your ex but, sometimes, walking away is the ethical, right thing to do.
I would echo Redpepper and River in examining why you had such terrible anxiety. One thing I've learned is that poly relationships will push one's buttons harder and faster than many mono relationships but the buttons themselves are often the same across poly or mono (or both) relationships.
It would be helpful for you to understand what happened - not in an effort to fix yourself or make yourself poly - there's nothing wrong with you! - but because that anxiety might indicate something going on internally. Or it might not - but at least you would know.
So why were you anxious? Were you actually jealous? Or fearful? Was the ambiguity of the relationship driving you a bit crazy? (Ambiguity is one of the things I find hardest to handle.) Was it that you wanted a monogamous relationship with this lovely man and that was just not possible?