Dealing with someone with Borderline Personality Disorder (BPD) is always incredibly hard and frustrating, not to mention painful. Being in your situation is difficult because of the dynamics of her being the 'ex' and having a child in the mix.
My advice to you would be to learn about this disorder, it is highly unlikely she knows she has it, or if she has been diagnosed that she will accept it... but you can take steps to protect yourself and your family by learning about her triggers, why she does the things she does and why she reacts the way she does, and then learning to detach and put in place strong boundaries.
My partner has BPD so I am speaking from a place of understanding, from being what I have learned to be affectionately called a 'Non'. A 'Non' is the partner or relative of someone who has BPD. And believe it or not there is an affecionate term for someone in your situation... a 'Non-non'...lol... a Non-non being someone who is now involved with a person who is a relative/spouse/ex of someone with BPD.
It is a very hard disorder to be around and deal with but please take heart that you are not alone in dealing with this...
.... I have found several resources on the net that have helped me...
I would suggest you take a look at this site for the friends/relatives/spouses of those with BPD, the message board has lots of interesting reading and tips on dealing, there are some non-non's there too ;)
www.bpdfamily.com
good luck
Bishy x