From what you've been saying, she may not be likely to start a divorce.
She has no reason at all to start a divorce. Her "complaining" that motherhood stopped her career" - this is her "rewriting history" to avoid shame and avoid her issues being discovered, which I assume hasn't happened yet with new GF.
The serial "pie in the sky" wishful and magical thinking - wanting the next thing- the new house, the place of her own, the GF, because she thinks this is going to be the solution for her discomfort- you've seen that and you've seen that if she attains it- in time, there's another wish.
Even if your wife has complaints, this situation is working for her. Only if this new GF is able to and willing to provide for her better, take on the role of emotional caretaker, and things progress to this point would she possibly consider it. Since relationships with pwBPD tend to be unstable, I don't think it's realistic to count on this if you want this. The more likely situation (IMHO) is that once "reality" sets in with this relationship- as it does for everyone to some extent after the initial newness, it would fall apart.
This relationship is like dating in high school. Parents pay the bills, there's no children to be responsible for, no worries about jobs, a mortgage, saving for retirement. The most responsibility a teen carries is usually schoolwork and a part time job. The relationship feels idyllic. Marriage involves work, childcare, it's a more mature commitment that teens aren't ready for. This is working for the GF too. She doesn't have to support your wife or share the care of the kids. They can have fun together while someone else does that. The reality of a long term relationship hasn't set in for them and it may be that the GF doesn't want that- she just wants what they have now. This situation doesn't necessarily lead to your wife divorcing you for this.
If you want to end the marriage, you will need to do that task but from your posts, it doesn't seem to be what you want to do at the moment. I think you'd be OK if she did it, but for you to do it, you'd need to take action. On the topic of boundaries- boundaries are what we act on. If infidelity was a boundary- that line was crossed when your wife went on a dating site. I'm not being judgmental or preaching morals- what two people do is between them and if both are OK with this, then it's not an issue in the marriage. It's that she did it and it wasn't OK with you but you adapted to it. It's not a boundary on which you will act- but boundaries are individual ones and so you can choose to tolerate this.
I watched this dynamic between my parents and it can go on for the long term. Dad mostly went along with BPD mother's wishes, and so, her wishes took the lead in the family. It seemed as if she could do what she wanted and without consequences, which isn't how the real world works, but it worked for her because this is what their arrangement was.
In every marriage there is the need for compromise and forgiveness, and to overlook some behaviors but also, people have boundaries- which are the very limits of what someone will tolerate. What behaviors are so intolerable that one can't remain married? Only you can decide what that is for you.