What you feel about being numb, mixed emotion, and grieving over not growing old together (grieving lost dreams)... .is completely normal... .regardless of mental illness involved. Divorce is hard regardless of BPD or any other condition.
I remember getting the papers from divorcing my non-BPD exhusband... .and I felt exactly the same. you will have days where you will revisit those feelings, especially holidays.
Thank god i didn't make the mistake of marrying my BPDx.

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Just beware of your rebound relationship. Take time to be good and solid and independent. The rebound relationship has been a doozey for each and every one of my friends and myself. It can sometimes be worse than the marriage (believe it or not).
Have you thought about getting into a divorce support group that focuses more on you- and not the mental illness, etc.?