Very sorry to hear the all-too-familiar story, AchingHeart. You have my empathy.

First off though -- get your cat back, dammit!
But seriously, though my circumstances were different I completely feel your ache. My ex had only separated from a 10-year plus marriage with her H in May and he only recently moved out of their house this February. Though she would always tell me she had no desire to re-enter the dating world and resume "playing the field"-- early in our r/s she revealed to me she had been unfaithful to her H for a good part of their marriage and thought of herself as 'single' -- I always felt that once she became truly "available" to me, i.e. separated, moved out from her H and divorced, that it would be likely that her and my r/s would suddenly become much less ideal. This was also before I new anything about BPD and F.E.R. (Fascination - Enmeshment - Retribution).
I don't believe she was unfaithful to me during our 14 month r/s and we spent much of our time together or with her kids. That said I wouldn't be shocked if she had been, especially towards the end. Early in the r/s (1st 3 months) a disagreement led to NC for a day or two and she said it was her usual pattern after she fell out with an SO to immediately hit the streets and start acting out with other men. She said however that this was the first time (after our temporary NC) she ever chose
not to do that and considered this part of her growth. This may have also been a ruse. As you know judging reality vs. lies/mirroring/fantasy/projection is hourly work in a BPD r/s.
The bottom line is I never fully developed trust in her and in
reality this is no grounds for a lasting romantic r/s
or a friendship (for me anyway). Even back then this helped me to understand that the fantasy we were hatching together never really stood a chance against the light of day. And yet I forged on.
So I would offer to you not to think of it in terms of "is there/was there/will there be somebody else?" but just realize that BPD is a
disorder that constantly seeks attachment. At some point the drive to attach to a new host for survival is as inevitable as the tide.
And while it hurts deeply to think of her being with another person, I think the pain I feel is tied up in something much deeper than that, because again, the
reality is that any intimate r/s they choose is doomed to the disorder, and why would that be something that I allow to create pain for
me?
How do you completely get rid of the fog? I have read the lessons and haven't really been through the anger phase. I got angry in the first week or so but that barely lasted a few minutes to an hour. I fear that I'll go over all these emotions again and I really don't want to. I'm getting to accept the situation although it is still tough. I'm trying to apply mindfulness whenever I feel down. I just want to get over this!
I'm still in the fog myself and slowly emerging. I do know that we
can't shortcut the process. Sorry, but no magic bullet exists.

As the charts on the right says "Attachment leads to suffering, detachment leads to freedom" with time as the ultimate arbiter. Once you realize for yourself exactly what you need (or no longer need) from the r/s then the process to begin healing starts in earnest.