I agree it's not linear. 3.5 years is a long enough time in an intense relationship, which it seems the ones with pwBPD are. I was with my ex for 2.5 years. Personally, I think many of us on these boards underestimate just how strong we are after N number of months. This is because it's precisely not linear.
I don't even know how to start quantifying the time. I guess most of us naturally start counting from the day we decide to go NC (breaking up, so to speak). But I know that in my case in all fairness I should probably date the "breakup" months before actually doing it. Several months, probably close to three or four, before I actually said goodbye. We went through the functions of being in a relationship. I stayed over, we cooked, we did chores, we had laughs on walks, but it was all fake. I don't know if she felt it was fake but I did. I didn't trust her in the sense that I didn't trust being myself with her.
My point is that throughout the final months I was in the process of breaking up slowly. I was detaching. I couldn't have ended it and then gone through that process. I grieved the relationship while still being in it. So it's from the point that you realise you can't be together that we should probably start counting.
I was with my pwBPD for 15 years. The first 5 were ok, but looking back, I now see the red flags I chose to ignore. Then things got crazy, but because I had the first 5 years, I thought we could get back to that and in my own codependent ways, I tried to fix it. The last year or year and a half I did the same as you... .detached and grieved the relationship I thought I had while going through the motions of the relationship. Calling it off was a huge relief and after that was easier than the period of time I faked it. As I said, though, there are still hard days, but I expect them and they are getting less intense and pass quicker as time goes on.