This is just my opinion and I apologise in advance if I'm offending anyone. 

I was diagnosed with BPD when I was 31 after being misdiagnosed with depression and anxiety when I was a teenager and a young adult. BPD felt right, I did a lot of research after the diagnosis and it was like "finally, someone understands me! This is what I've been feeling since I was a teenager!" It was a relief to have the right diagnosis.
I have been making progresses, at least according to my therapist. My therapy is MBT and it seems to be working, except for relationships and anger management.

Why this whole explanation? Because yesterday, while having a normal conversation with a friend about life and death, they said: you're so full of rage, why are you always so full of rage about everything?

Because I have BPD! Why can't people understand this simple fact? Rage is one of the characteristics of my mental illness and I've always wondered why I never became violent towards people. Maybe it's because I tend to turn violent against myself, like self harming, because I need to be punished for feeling that rage.

But why won't they understand? It's an emotion, a deep, raw emotion which I cannot control even though I try.

No one seems to understand no matter how much I explain. It's painful.