You may want to try posting in the Poly Relationships Corner section, that's where more people post I think - I usually read that one and the General Poly Discussions section, not so much these others.
To sort of answer some of your questions - yes, people can love more than one person at a time, and have long term happy relationships.
No, those long term happy relationship don't have to be in a "triad" formation, which is what it sounds like you were trying to do and your wife is saying won't work.
I suggest reading a lot more on these forums; I'm relatively new here, but I've learned a TON just by reading other people's stories and just listening to what the more experienced people say.
But the biggest thing I think would be to slow down, discuss things with the wife and the other lady, and figure out if you even want to try a polyamorous relationship, if you all want to learn to work through jealousy and any other issues, etc. Get more information and get everyone on the same page, so to speak.
Guy - Dom/husband
Minx - sort of dating
Cru - (no longer dating)
Lee - Guy's girlfriend