It seems like at times things get moved to "new to polyamory" because the topic seems to have something to do with a new poly type "skill". I know that I had a thread that went to this section having to do with dealing with NRE, even though I've been non-monogamous for 15ish years...
On the original topic... perhaps it might be helpful to discuss with your metamour what her original issue was that prompted the veto. If you express concern both that you don't want to be vulnerable to having your relationship ripped away from you AND you are concerned that there were things going on that were hurtful to her that you were unaware of and would like to avoid that in the future-- that might give her some feeling that you are willing to work WITH her on making sure everybody's needs get met.
I'll qualify that recently I was in the position where I SO wanted to tell my hubs to give up the GF, but knew I couldn't, shouldn't and wouldn't. But at the time he was so wrapped up in NRE and neglecting our relationship completely, not listening to me, not hearing me, running over boundaries left and right and basically just doing whatever he wanted that I was about two steps away from leaving him. And all of my communication about what was going on was completely ignored, so it wasn't like I wasn't communicating the fact, he just wasn't *hearing* me at all.
I know that some of this (perhaps most of it) his GF had no idea about. Because he was texting/calling her all of the time she assumed he was doing the same for me. Nope. I basically lost almost every bit of contact with him because he was so enthralled with her and was entirely incapable of carrying on two relationships at the same time.
It would have been easy for me to blame her, or the fact that he was WITH her for the pain I was feeling-- because it WAS caused by that relationship happening. But it was caused by HIS behavior within that relationship, not because of anything she did.
Anyway, just wanted to give another side to the story of why someone may panic and pull the plug. For me, if I knew that my husband's other partner was being well aware of the NRE and making sure it wasn't getting out of hand and they weren't rolling all over me would have been really beneficial.