I experienced a co-founder split not too long ago and I can definitely tell you from my personal introspection that you may need to look deeper than "it was my fault for not seeing the warning signs".
It always takes two to tango and you're right to not bitch about them but then saying you were stabbed in the back is silly. Being a founder is tough, having co-founder problems is tougher, clearly understanding what your part in the dynamic is or was is even more difficult.
Sometimes, too, to get this level of internal depth takes distance from the people and the experience. It took about six to eight months for me to figure out what was rightfully my fault and what was rightfully my co-founder's fault - beyond "seeing the warning signs early".
Also, I don't know what your captable looked like but if you were the controlling interest you cannot and should not expect anyone to care about what position you're left in - you must be prepared to shoulder the burden on your own for a bit if they decide it isn't worth their time anymore. The more you own it the more you own it.
Hope you're okay and sorry if this comes off as admonishment, it actually comes from a heartfelt place. You're welcome to contact me personally if you want more support.
[EDIT]
Re-reading my comment, I might be coming off too much as an armchair psychologist, I'll leave the comment up for posterity and maybe you'll find something useful in it or not. Either way I think you've handled yourself well and hope that you yourself are doing well.
1. Even though I'd already spent 2 years on the startup, the first co-founder I bought on had the same equity as me. He wanted it, and I wanted someone fully committed.
2. I was off fundraising for 2 months, and my mistake was not communicating that really well with them. I wasn't a great leader at that time. However, I did notice something was off but I just thought it was post-yc depression (which is actually a thing, apparently). I asked them outright if everything was ok, and they said sure, that it would all be fine when we got back to Australia and moved on. I trusted them, and they lied to my face.
3. I think they were very naive, not malicious, in how they left. They thought everything would be fine if they just left, but thats not how startups work.
4. One of them refused to talk to me or give me an explanation after he left. I had to practically force him to meet me for some closure. Its like getting a divorce with no explanation.
I made mistakes too so its not all on them, but at the end of the day a co-founder is supposed to be your partner through thick and thin, and they bailed without talking through the issues at all which equates to stabbing me in the back. And they made sure to tell me this after I'd reimbursed them for thousands of dollars in expenses for their YC adventure.
The purpose of this is NOT to discredit them. I'm not a huge fan of them, but they dont deserve to be villified. I'm just trying to be a bit more transparent about what happened so that its helpful to some readers. I'm sure they've learned from the experience and will treat their next cofounders differently if they do start something else.
In progress on a startup now, and I have realized the most important thing is trust between the cofounders. There is a lot of people who want to be 'cofounders' but don't understand what it means, the real consequences and also what real company control means.
I also think YC might not be helping here - since being a YC founder is such a cachet resume item, the value of growing a company is discounted. This results in real problems as you have illustrated.
In the end, if you don't trust and know your cofounder as well as your SO, it's going to make the hard things that much harder.
I was in the same situation. In my case the reason why the co-founders left wasn't told was because the reason was that they thought their partner was too naive and incompetent to work with. Not something so easy to say straight to the face. In hindsight, I think there was no wrong in feeling betrayed in my case, but at the same time the co-founders did no wrong either. We were too inexperienced to handle the situation gracefully. Break-ups can't be amicable all the time. Everyone has their side of story.
The point went over your head, in any failed dynamic there are always mistakes made on both sides. I'm trying to encourage her to think deeper (not write) about it beyond not catching the "warning signs" which places her in a victim position. Whether she was or not, victim cycles create repeat patterns until the person sees their own role in it.
I'm encouraging psychological healing. I've very much been through a very similar experience.
[EDIT]
It actually sounds like it turned out well since she did end up with a solid co-founder relationship anyway. I'm leaving this up for posterity but now think my thoughts on this are unwarranted.
It always takes two to tango and you're right to not bitch about them but then saying you were stabbed in the back is silly. Being a founder is tough, having co-founder problems is tougher, clearly understanding what your part in the dynamic is or was is even more difficult.
Sometimes, too, to get this level of internal depth takes distance from the people and the experience. It took about six to eight months for me to figure out what was rightfully my fault and what was rightfully my co-founder's fault - beyond "seeing the warning signs early".
Also, I don't know what your captable looked like but if you were the controlling interest you cannot and should not expect anyone to care about what position you're left in - you must be prepared to shoulder the burden on your own for a bit if they decide it isn't worth their time anymore. The more you own it the more you own it.
Hope you're okay and sorry if this comes off as admonishment, it actually comes from a heartfelt place. You're welcome to contact me personally if you want more support.
[EDIT]
Re-reading my comment, I might be coming off too much as an armchair psychologist, I'll leave the comment up for posterity and maybe you'll find something useful in it or not. Either way I think you've handled yourself well and hope that you yourself are doing well.