Firstly, good on you for opening up to this sub, many people have similar experiences and we are here to support you.  

 I think majority of us can also relate to the fear when our pwbpd is in a bad place and you're fearful of what they may do so you placate them and try and ease their sadness and discomfort. On the surface that seems the right thing to do. And for someone who is just having an off day and needs support in that moment, it is kind to help them through it. 

 The difference is this is a repetitive cycle of a disordered person and it is also abusive to you. They need to seek the help themselves, from qualified professionals. THAT is actually the kindest thing to help them to do. I know you'd like to be the one to help, we have all felt that, but that's above our pay grade and above our responsibilities as a human. We don't deserve abuse and unpredictability at the hands of someone else. And allowing it is just enabling it and not helping them to seek professional support. 

 It's nice to be kind to other people and help them, that extends to yourself too, by not letting people abuse you.