First of all, I just want to say that there are puhhh-lenty of people in their 60s and older with wonderful sex lives, with totally hot, frequent sex. I'm just sayin'.
Next, I just want to ask what happened to the friend of yours you got to experiment with before your SO told you to dump her. Is she still a friend? if I were her, I would not be. It's rather cruel to experiment on other people, test poly out, while letting your partner call the shots for relationships she is not in. If I had been treated like that, man, I would be pissed and tell you to grow a pair.
Lastly, I would say that I don't think either of you are ready for poly. That doesn't mean it can't happen. But basically it sounds like you two had a few talks, decided it was okay for you to have a fuck buddy, and then jumped right in. Some couples take a year or more discussing, soul-searching, strengthening their relationship, looking at possible glitches. I think you may have just gone into it unprepared. Now you know what one of her triggers is, but it's up to her to deal with her insecurities. Maybe you two are better off breaking up, who knows (it's not the end of the world if that happens - most relationships aren't meant to last forever), but I think you are both settling and unhappy. I also would say that any big changes to your relationship need to happen slowly and with more forethought and consideration - for everyone you may get involved with, not just your long-term main squeeze.