If you are planning to divorce your H, it's a good idea to gather as much information as possible before you tell him. Work out what your goals are, and then think about strategies to get there, keeping in mind the specifics of how your H's BPD expresses itself. Usually, whatever you experience in the marriage, you are likely to experience the same if not more in the divorce.
I agree with the above, work on developing your plan. Make copies of any important documents... .car, property, birth certificates, bank statements, pay stubs, insurance information etc. What are your joint assets? How would you like to see them divided? Do you need to protect sentimental/valuable items that have belonged to you from pre-marriage? Who will live where? Custody?... .
As far as Mediation I would just GOOGLE Mediation in Northern CA see what is in your area then just give them a call. Do you think your husband would negotiate in good faith? You might also want to consult with an attorney about what your other options are just in case. Do you want/need an attorney to represent you, even in mediation? Do you feel that your husband might be able to bully or guilt you into giving up things you shouldn't?... .things to think about.
In my SO's case mediation was not productive his uBPDxw would agree in mediation to things and then back peddle, but every situation is different. Just try to be prepared for as many possibilities as you can.
I also encourage you to read "Splitting... ." by Bill Eddy. You might also want to review some of the other posts on this board for other's experiences, strategies and ideas. One thing you might want to start is keeping a journal on your husband's behaviors as documentation (keep this information well hidden), emails and texts can also be good documentation.
Don't let all of this overwhelm you just take it a step at a time and don't forget to take care of you during this stressful time. Once you let the cat out of the bag to your husband, be sure to reach out to friends and family for support.
Take Care,
Panda39