New marriage rapidly going bad, really want to heal things

[SleeperService]

Hi, first post.

I'm not sure how to put all this so I'll just dive into it. My wife and I got married ~6m ago after a short relationship that began in very difficult circumstances. We're both in recovery from severe addiction and she was actually mid-relapse when we first got together. Since then we've both been clean+sober bar a couple of small lapses on her part. We've been living together since we got married, slowly getting back into work and normal life again.
I knew when I met her that she was a very chaotic person. Her behaviours back then were shockingly impulsive and risky even by the standards of someone in active addiction. I went through a lot sticking with her until she got clean again, after which we got married rather impulsively with the intention of leaving all that behind us and trying to live a normal, peaceful life together.
Things were great for the first few months. We would have occasional massive blow-outs but they never lasted long and were soon mended. We both have our share of diagnosed issues; bipolar II in my case and whopping ADHD in hers, so you can imagine how well they mix sometimes. It's become clear that she almost certainly has BPD, as well. It runs in the family and all the signs are clear as day, but as she works in mental healthcare herself she's almost morbidly afraid of the diagnosis due to the surrounding stigma and poor outlook it carries. In truth, she knows perfectly well she has BPD but clings to the idea that it's just a few shared traits with ADHD and panics at the suggestion it might be anything more.
So anyway, to begin with we were 90% madly in love with the odd gigantic fight now and again. We both remarked lots of times that given our m/h issues we actually did extremely well, both having some insight after spending much of our lives self-managing without a lot of treatment. We were both good at going over where we'd gone wrong and understanding each others' fears and triggers. I think we were both willing to just accept that it would be hard sometimes but felt like we had a handle on it.
Then a couple of months ago, roughly coinciding with my son receiving a cancer diagnosis, things just began falling apart. My mental state took a turn for the worse and I started becoming disorganized and erratic. Up until that point I'd very much been the caregiver in the relationship; we were both well aware of it and spoke about it openly. She needs to feel cared for and I need to feel useful so we decided to just roll with it since we were both fulfilled. As soon as I started to slip and make mistakes, though, she began splitting on me constantly. In the space of a few weeks she went from dreamily content to constantly telling me that I'm useless and she regrets ever getting involved with me. Things have switched so it seems like now 90% of the time she's absolutely furious over things I've forgotten to do or perceived slights, while the other 10% she's apologising and looking for reassurance that I still love her. It's honestly kind of frightening; as soon as something trips that switch in her she's like a different person and nothing gets through, her face even seems to change. If I try to remain calm and talk her through it she takes it as cowardly and dishonest. If I fight back she just blasts me with resentments and contempt until I can't think straight enough to answer. For my part I always end up making things worse by getting scared she's going to run out on me and relapse, so I follow her around dragging things out even though I know it's the wrong thing to do. My mental health is never great but I'm getting to a real crisis point dealing with fears over my son while at the same time she just seems to hate me more and more for the perceived weakness.
Having read around a little bit this all seems like absolutely textbook BPD stuff and my heart's breaking thinking that we're just going to go through some generic cycle until the marriage is ruined. It might not come through in this post but I love her very deeply. I might have blinders on but I honestly don't think it's just lovebombing or infatuation. Everyone who knows us seems to think we were made for each other. When things are good it's not just the kind of frantic idealization I read about, we truly love each others' company and thrive in it. We'd both been doing better in life than we ever had before and building each other up. 
I'm at a loss for what to do because things have become a vicious circle. I tried explaining last night as kindly as I could that I'm not going to get any more reliable or stable through being abused but all I got was more of it. I still feel 100% committed because I know she's just unwell and we can be great for each other but it feels like it's all slipping out of my hands. I don't know whether to try and scrape up some money and suggest paying for DBT for her, or what. I know I should detach a little and let things calm down but she's so vulnerable, I'm terrified that if she took it as rejection she'd start using again and I'd lose her. I'm willing to try pretty much anything.

[RELATIONSHIP PROBLEM SOLVING]

RELATIONSHIP PROBLEM SOLVING This is a high level discussion board for solving ongoing, day-to-day relationship conflicts. Members are welcomed to express frustration but must seek constructive solutions to problems. This is not a place for relationship "stay" or "leave" discussions. Please read the specific guidelines for this group.

[Cat Familiar]

Have you looked at the Tools section at the top of this page?

The biggest takeaway I see in your post is that you are trying to bridge the communication gap with her when she has become dysregulated. Sad to say, it’s pointless at that time. She needs to go through whatever it is that she’s feeling and is not available to hear anything from you until her emotions have returned to an equilibrium.

Trying to “talk her through it” registers as demeaning, and fighting back just furthers the conflict.

Probably the first thing to try is to not JADE (Justify, Argue, Defend, Explain). That’s a very difficult lesson to learn. Though these strategies can be effective with someone who is emotionally healthy, they are completely toxic to someone with BPD.

[SleeperService]

Thanks for your response. I'll try to avoid doing that, but I admit it's difficult. Strangely she actually raised the subject of BPD yesterday during a calm moment without me saying anything, but then as the conversation continued she flatly denied having any traits and became angry/blaming of everything towards me again. I'm hoping it's just too soon and that if I can follow the advice on here things will stabilise enough that she can face at least considering some changes.

[martyme]

sorry to hear -sounds ridiculously stressful!

Seems like the struggles started with your son's diagnosis.  Would it help you and your son if you fully devoted yourself to your son's care/enjoying time with him? Don't know how old your son is or if your son/wife have alternative sources of support, but i'd guess that your priority should be to your son, especially as anyway seems that despite your best intentions, you cant help your wife at the moment. You may find focussing on your son works best for everyone - it relaxes you, helps your son,  and encourages your wife to miss and appreciate you.

I'm no expert -so keen to hear others thoughts.