It is unfortunate with child support, you have no say in how it is spent. It must be very difficult to hear from your kids that they aren't eating well. :'(
When I'm feeling like, why me, why did I have to meet him? Why couldn't I have met a nice loving person? Well, for some reason, I wasn't interested in the nice loving person. Something in me was attracted to the BPD type of person. I wasn't so much a victim as I was a willing participant.
Learning about BPD helped me to start looking at my own stuff and working on my healing. This sort of work might be helpful to you and in the long run, to your kids, too.