I found !bpd@sh.itjust.works and !bpd@lemmy.world
They are not very active, but the first one have a recent post. I think you can post in the mental health community and find support and tools too. There may are others communities.
I encourage you to take BPD diagnosis with a grain of salt, and to be careful about it, depending on where you live. While it can be helpful, it can be a heavily stigmatized diagnosis too, even by health professionals.