Well - I'm doing quite a bit better, GF sometimes seems better, sometimes worse. She asked me again if I was OK if she sees the mono guy again. I still reacted emotionally internally (though not as powerful as before), felt that pang of insecurity, but was able to tell her she is completely free to see anyone she wanted, but that I felt uneasy that it would eventually hurt her again. I suggested she try to be aware of how the relationship made her feel. Now, I'm letting go of the situation; my GF needs to find her own truth in this. Its hard to sit on your hands when you see someone you love heading in a direction where hurt is pretty likely. Thats actually a stronger feeling than the minor insecurity I feel, but she has to be free - even to touch a hot stove - but who knows, maybe the planets will align, and they can have a healthy fulfilling relationship.
The funny thing is that even though I haven't met the guy, I like him based on her description - I wouldn't mind meeting him if it wouldn't be too awkward for him.
After all this, I doubt I will date anyone monogamous - there are just too many ways it can go wrong.
I think I'm still breaking out of a monogamous mindset myself - all of this stuff has been focusing all of my attention on one person - and that is not helping; I met someone a few days ago that seems really cool - I'm starting to think the healthiest thing I can do for my GF is let her be for now and worry about myself.
The truth is that I want her to be completely free to experience and explore romance - this is what I want for her. Its just hard to see her hurt in the process.