I have been diagnosed with BPD for years and I am now suspecting that my partner also has BPD. We have been married for a few years. 

Recently they had seemed to have had an episode after I had done something to hurt their feelings unintentionally. I apologized, however, they were really mean about it and said things like how I always ruin things and make it impossible to have a good time. I expressed how that hurt me as well. They apologized, but continued to be cold towards me. Their demeanor made it really hard for me to "get back to normal" and they really hated that. They didn't understand why I wouldn't just get over it and it just continued to get worse. 
They started to say more awful things like how I was embarrassing and selfish. How they couldn't stand me anymore and threatened to leave multiple times. This entire time, I'm just feeling suicidal and crying uncontrollably. I just feel so awful that I can't even really get any words out except for please and stop. After what feels like an eternity of awful words, it's like a switch flipped and they felt incredibly guilty for everything that was said. They started profusely apologizing, trying to self harm, and begging me not to leave. Watching this made me feel like I was looking into a mirror almost. When this happens, I put my feelings aside and try my best to comfort and support them because I know how it feels. I just wish I had the same energy returned sometimes. I would never say such awful things even if during an episode I definitely want to. I have discussed with them on many many occasions what BPD is, what it means for me, how it feels from my perspective, and how to support me. Not much has changed unfortunately. 
Anyway, this is definitely not the first time this has happened. They have other symptoms, impulsivity, self damaging behaviors (substance abuse, disordered eating), splitting, mood swings, etc.

I just want to know how I can go about bringing up my suspicions and how to support them as much as I can. I love them dearly.