So I'm writing on here because Google isn't helping me out much and I don't know anyone with BPD who is in a long-term relationship that I can ask about this. 

Basically, my issue is that I can go from being completely fine and happy and enjoying being with my boyfriend one minute, and he will just say one thing and my mood completely plummets; I'll feel like I want to die, that he's better off without me, that he doesn't love me the way I am and that I will never be able to move past what he's said and be happy again. I want to run away and stop existing. 

Normally the things he will say is just a slightly insensitive comment or something he's phrased wrong. I know he loves me and I know he understands my mental health (the best he can - he tries his hardest) and I know he doesn't mean to hurt me. 

But somehow these rapid 'spirals' always happen and I go to the absolute extremes of dark places in my head? And when it's over - often after I've felt shit for a couple of hours (sometimes longer, sometimes shorter) and we've had intense arguments - I just suddenly feel fine again, and I can't relate to how shitty I felt just moments before.

Is this splitting? Or is it just something else? 

I hate that it happens because it leaves its scars in my head and is just utterly exhausting and I don't know why or how it happens most of the time.