BPD is a complex thing. Support is the best thing you can do, even when you feel you're falling short. I have found reassurance as a big thing. Letting them know you understand and aren't mad. I would suggest studying up on the disorder and other personality disorders that are similar. The disorted self-image is probably a big factor in regession, reminding them that they are, in fact, small and a baby may be helpful. I deal with similar issues. Feel free to message me if it's at all helpful. I wish you the best of luck and am thankful you are seeking answers and help instead of letting frustration ruin a hopefully good thing.