Welcome!
Though the reason for your arrival is not a pleasant one, you truly have found others who have walked in your steps in the past yet have found ways to deal with the cycling craziness. Take advantage of the collective wisdom we've gained in our years of peer support.
You mentioned both Borderline PD and Bipolar. These are different but do exhibit similar behaviors. Bipolar is largely a chemical imbalance but meds to help. On the other hand, BPD is a mental health illness that impacts perceptions and the close emotional relationships. So partners and children are impacted the most. While meds may moderate the BPD behaviors somewhat, the best approach is intensive therapy from an emotionally neutral professional (a therapist) for years. The challenge is that people with BPD (pwBPD) are in Denial, Projection, Blaming, Blame Shifting and cycling between push and pull behaviors.
I feel like I need to make a plan to leave and prepare for divorce so I don't end up paying alimony besides I don't trust her with the kids... But to see my two daughters crying and getting anxiety over this makes me realize that it's not about me anymore it's about them and I need to think about a life without her...
It would be smart to quietly seek interviews or consultations with a few experienced family law attorneys to determine where you stand as an unmarried father with children approaching adulthood. We we say "quietly" this means don't share what you learn with your partner... it is
private and confidential for you. Why not tell her? Well, do you recall the old wartime saying, "Loose lips sink ships"? If you share too much information (TMI) with her then she may find ways to sabotage your goal of finding relief for yourself and your children.
Am I crazy for thinking of staying? She love bombs me for a few days and then splits on me constantly. What are the negative implications of staying?
The implications of staying will be continuing stress and chaos on both you and the children.
A few decades ago the book Solomon's Children - Exploding the Myths of Divorce had an interesting observation on page 195 by one participant, As the saying goes, "I'd rather come from a broken home than live in one." Ponder that. Taking action, as appropriate, will enable your lives or at least a part of your lives to be spent be in a calm, stable environment - your home, wherever that may be - away from the blaming, emotional distortions, pressuring demands and manipulations, unpredictable ever-looming rages and outright chaos...
Likely it would be up to family court to determine the details of how much time is spent by the minor children with each parent (until they become adults and age out of the family court system) but you as the reasonably normal parent need to seek as much time and authority as possible in custody and parenting schedules.