Hi peaches and welcome!
You are in the right place to get support and help with figuring out better ways to communicate with your fiancee. We get it here and have people posting who are new to this or have worked through and learned a lot of the lessons already so you are in good company.
Can you tell us more about his behaviors that make you think he has BPD? More details will help us get a better handle on things so we can better guide you. We have lots of tools and strategies that can help quite a bit.
I hope you settle in and read and jump into other threads as we all work together to support each other here.
Again,