Hi Vent,
I am so sorry for your break up pain! I've had some painful breakups myself. They can really knock the wind out of you - whether you initiate them or not, whether they are with someone with BPD or not.
I'm glad you have the support of a therapist and that you can spend some in-between here posting. I quickly skimmed your past posts, but I am not totally clear on what happened yet... .just wanted to focus on on these feelings you expressed here.
It is a good idea to find other things to fill your thoughts. When we ruminate it can pull us into the past and make the pain more acute, but you can't just toss this all aside either.
Your ex moved on pretty fast after the breakup? I had an ex who did that too. Heck, I think I even did that once (and I'm not BPD) just to make myself "immune" to going back to an unhealthy situation. But when my boyfriend with BPD traits suddenly left I was floored! Floored! I am almost embarrassed to say how low I was at that time. But if there is one thing I can promise you it is this, time will heal you. Be patient with life and with yourself. Stop now and try to picture your life in 3 months and 6 months. So much can happen in just that amount of time! Let your mind go a little into the future and not back to these past memories.
Maybe this notion will help. I heard it years ago. Thoughts are clouds and mind is sky. So lay back and look up at the sky and all those thoughts/clouds drifting by. You can't hold onto clouds right?

So just watch them and let them go. More clouds will come. Dark ones and light ones. That is life. But then look at sky! How big is sky?

You are so much more than this breakup. Life has so much more in store for you, okay?