Hi jadedcat!
I've been out of our house for 10 months, so I know what you're talking about pretty darn up close and personal myself. It's really really hard. We have to grieve the loss of all the things we thought we were going to have as a couple, and slowly over time realize how many of the loving things our partners say are about getting their needs met rather than for any concern for us. You're right, it's an illness, and that makes it seem so sad that we have "given up" on our commitment to them--but we haven't actually given up on them, we've finally realized that we need to care for ourselves and our own mental health.
The analogy is used around here of the airline attendant doing the safety talk, and how she'll say that if the oxygen masks come down to put your own on first before helping your child or anyone else, because how much good will you do anyone if you're passed out on the floor because you didn't put yours on?
So many of us who get into these relationships have huge hearts for people and we feel awful when we can't fix them--but that's their job, to fix themselves. To see where their relationships aren't working and make the decision to get help to change the things that need changing. We can't make them choose for their mental health. That was so hard for me to accept as well.
As far as friends, I found the same problem with long time friends--they couldn't understand and haven't been very sympathetic, but I've been fortunate to have some newer friends through a writers group who can just feel my emotions with me and be there for me.
It's a big, mixed-up mess of emotions that we feel in these complicated relationships, and one of the best things I've learned in this time is to sit with those emotions. I can be really clear one day about why I've left, but the next day be sad about needing to leave. It's all part of the grieving process, and as hard as it is right now you will move through it.
Weirdly for me, since I loved decorating for Christmas, when I go visit the doggies at my house while my uBPDh is at work, I hate seeing how he and my son have let it fall into such shabby shape, and there's no Christmas anywhere. It's just a sad, sad house without me there. Either way, it's gonna pull hard at our heartstrings, especially after years together. I do really feel for you in this time. My uBPDh keeps acting like it's all gonna be okay eventually, yet he still treats me with the same disregard and control and mean words he was all those months ago, even though he's been in counseling weekly for these past 10 months.
I like to write out my feelings of the moment, then when I go back I can see where I've moved through some particular part of the process. Do keep in mind that grief is a sneaky ass, and can surprise you with its intensity out of the blue. But know you won't feel that way forever. And that some days you may feel all the parts of grief at once, and they really don't come in a tidy order. But writing things down does show you the growth you've made over time.
Guilt is just part of that FOG we get covered by with our BPD loved ones--the Fear Obligation Guilt. I don't know if it's intentional when they cast that FOG net over us, but that doesn't really matter. What matters is that we are finally treating ourselves with care, and our mental health matters too, and it's the only mental health we can really have control over.
Try to take a deep breath and walk out of the FOG and into a lovely Christmas with your kids who love you and want you to be healthy.

baby steps... .