Hi Kurious Kat and welcome!

Being in a relationship with a person with BPD (pwBPD) can be exhausting so it's good that you're reaching out to others who understand and can give you support.
There's lots of information on these boards that can help, from the tools and workshops to others' posts, so spend some time reading and posting to gain skills and perspective.
Now that your fiance is diagnosed, is he getting therapy? Are you receiving any therapy to help you cope?