"What have I done to deserve to fall in love with someone with this disorder?"
Two major things often influence this, these combine to make you take the bait that others would pass over.
Deep down we are often at a time in our lives where we could do with a little validation, whether consciously or subconsciously. A pwBPD craves validation, so they sense this and immediately "bond" with it, they then pour feel good validation on you as that is what they want back themselves. To them validation is important. They then fill you with tales of woe, so that you then put on the rescuer hat (which also validates your sense of worth) and start validating back.
This is the basis of the early bonding and you are hooked.
Your need for validating is soon satisfied and you start then wanting a normal balanced RS. Their need for validation is not sated, it cant be, it is bottomless. Dramas and crisis are then created to try to draw more pitying and validation. The whole thing becomes unbalanced and the rot starts.
To survive this you have to disengage from the dramas and realize a lot of the early behavior was merely them mirroring what they thought you needed. Now it is about what they need.
They need you to be there as a rescuer, so all sorts of promises will be made to ensure you dont leave after they dump all over you. You dont, so this is the new acceptable standard. They dump on you, promise not too. You stay. The cycle is then reinforced as normal. Owning they have a problem does not fix it, as that then becomes a valid "excuse' and you are even more guilted into forgiving. You are feeling as though you are getting to the core of the problem, light bulb moments, but at the same time the difficulty in distancing yourself becomes harder as you accommodate them more as they appear to be trying.
The resentment and feeling of entrapment increases, and you can start to feel ashamed for allowing this to happen, but loyalty makes you stay because they are "trying". Pity also becomes part of the hold.
How to break this cycle? One way is start to separate your stuff from their stuff, so that you have your own independent self, and depend less on them.
Do not be afraid to leave then to deal with the consequences of their own behavior, rather than protecting and covering for them.
At the end of the day, all this is easier said than done, so dont beat yourself up when you fall short, we all do.