igotaway I am so sorry to read what a distressing time you and your son are going through, I can only imagine what pain you are enduring at the moment. I have attempted to write this reply to you a few times but I am finding it really hard.
Firstly, although people with bpd often have similar outward behaviors, everyone experiences it differently so my responces to your questions may have little relevance to how your son feels. I feel very wary of giving advise as such, as other sufferers I have met in hospital have needed different help to me.
"what did you want your mother to do for you? what didn't she do that you thought might have helped?"
I guess I wanted my mum to acknowledge my illness as an illness. I wanted her to read about it and talk to me about it from an informed point of view. I wanted her to acknowledge just how difficult it was for me even if she couldn't really understand. They were always so angry with my behaviors which just compounded my feelings of guilt and shame. They never tried to understand why I behaved the way I did or have any sympathy/empathy. I felt huge pressure from them even when they were trying to help me into uni/work etc as they thought it would help, their time would have been invested in helping me recover. I spent most of my days desperately wanting to die and they would go on and on about job applications! They underestimated how it consumed me as I appeared to be 'coping' a lot of the time. Even if it doesn't seem like it, emotional difficulties can be there all the time.
I would have loved for their help navigating the nhs, they never spoke to my doctors/cpn etc so it took a long time for me to get the therapy I needed. I would have loved to be able to discuss the practical things I had learnt in therapy with them and practice changing my behavior. A lot of research suggests DBT is the most succesful therapy for BPD. This involves practical approaches to dealing with overwhelming emotions and i think it would have been helpful to be able to talk about these outside therapy. The more time I am able to spend in the present i.e. not listening to my internal voice worrying about past/future, the more I am well. My parents could have helped with this if they understood, by prioritizing providing opportunities to do this. Just taking me out to the cinema/ going for a walk/ cooking with me/ taking me shopping could break the cycle of torment in my head. When unwell it is difficult to motivate yourself to do this even if you know it works.
I wish they could be open and frank with me about how I felt, and about their feelings. They tried to hide everything difficult as they didnt want to 'upset me.' I didnt go to my grandpa's funeral as they didnt think it was a good idea. This compounded the isolation I felt and reiterated that I was different to everyone else.
Mostly I needed constant reassurance that I was loved and wanted and they weren't going anywhere.
I know this is long and waffly but it is extremely hard to articulate. When I am ill now my CPN always asks what can she do, how can she help. The most frustrating thing is that in crisis I have no idea. It often feels like nothing/no-one can help. My guess would be that your son doesnt know how he can get better or how you can help. There is a book called Overcoming BPD, A Family Guide for Healing and Change (Valerie Parr) which is very good. I could send my copy to you if you think it may be something you would benefit from reading?
"What was going through your head when you were at your lowest?
How did you get back up again?"
Again it is really hard to articulate. At my worst my thoughts are racing and relentless, I can not switch off from them. I relive every failing and a torrent of self abuse goes round and round my head. Im worthless, pathetic, a failure, fat, ugly, lonely, hated and not made for this world. I conclude that I am better off dead, my suicide would be a good thing for those that know me. Then thoughts of suicide whirl round my head, I consider different methods. These though patterns can last hours/days/weeks and there is no escape. It is exhausting yet I dont sleep. Nothing seems worthwhile, life seems meaningless and pointless. I feel like I have no real connection to anyone else. I question my identity, even my sexuality. The thoughts keep coming thick and fast. I close my eyes and beg that I dont have to wake up as me ever again. I deeply hate every atom of my being. It feels so intense it does swallow me and I cant attend to any other thought.
The emotions when at my worst are so overwhelming, I am not surprised now that I have acted into them so many times as many sufferers do. Self harm, suicidal behavior makes sense as it gives relief from the pain even for a short time.
I have come back from this place so many times as a tiny part of me wanted to live, just not like that. I have come to learn that the most horrific times do pass. I know it is of little use when you are in the thick of it but with therapy and time you begin to trust that you can endure the worst and there is a future at the other side. There is always hope.