Paperinkart, I know it can be frustrating to feel you are stuck in the same place. Indeed with time the constant uncertainty will wear one down.
Is your partner in therapy?
For me what I found a priority was to learn to recognize when dysregulation happens. It is stressful when what seems like a perfectly ordinary conversation goes awry, and so to spot the signs early really helps.
Using the relationship tools at that point can often calm the situation. A good place to start is here:
https://bpdfamily.com/content/ending-conflictThe threat of abandonment is at heart a request for reassurance. When it happens I talk firmly about our future, how happy we are going to be together and how important it is that we work our way through whatever the conflict is. I used to get very upset but have learnt that I should not take the threat too seriously, but to look for what motivates it. Since I started doing this we have not broken up. I joke with myself that if she ever was serious about breaking up she would find it impossible to do since I would just assume it was dysregulation and ignore it
Once you have calmed the immediate situation, it may be worth while addressing the underlying issue. If he is lying it means he does not trust you or is afraid of you. BPD at heart is a disease of trust. I don't know what one can do about it other than be constant and consistently trustworthy, but in a calm moment it might be possible to ask him what you can jointly do to build trust?
Do check back in and let us know how it is going? Browsing other people's stories and conversing is also helpful, we are a community here to support one another.
hugs,
Khib