rawshark - what do I do when my dp hypothesises negative outcomes - good question, it is a really tough one!
Honest answer is that it varies - and how I am at the time is a big factor. I do not see myself as (and would not be capable of) being his therapist. It is, btw, a bone of contention between us that he has always refused counselling or therapy. I had counselling for 2 episodes of depression, and it helped, partly by making me more aware of my own thought processes and how mood affects them. Dp struggles to accept that the way he perceives things and how he reacts are informed by his depression, and he over intellectualises things to try and prove he is 'right' and I am 'wrong'.
Most important thing tbh is not to get dragged down by him when he is like that. He doesn't want to reflect on his own thought processes and decision making - he wants to argue with me about mine and thus seek confirmation in his doomy predictions! often this means just not engaging with him - which is hard, and counter intuitive, but necessary. For self preservation, really.
If I think i can stay calm and focused, I will ask him to talk me through his reasoning - that series of thoughts that leads to 'I'm not going to do anything because it will all be awful and go wrong.' This is VERY hard, as he is very unwilling to articulate his 'process'. When he does, so often his assumptions sound daft - because they are daft - and it becomes clear that he is arriving at decisions and forming opinions based on something that in the clear light of day, just doesn't stand up. You'd think this would be a good thing! however... often his reaction to that is to get very angry, as if he has been 'exposed' (which he has, in a way) or very down ('I'm just rubbish at everything then' etc). And quite often I will blow up along the way anyway, as some explanation for why he has/hasn't done something finally comes out, and I find it ridiculous and very frustrating.
A typical small example - he won't talk to people or ask them something because 'they'll probably say no'. I say 'you don't know that - you need to ask them'. He says 'but they'll say no'. I say 'you haven't tried'. Eventually he might explain WHY he thinks they'll say no (and this takes age s to get there) and it is something so flimsy it is basically made up. As he says it, it sounds thin, he sort of panics, especially if he has been very insistent he is right, and then goes into 'alright well I just can't do anything then!'
At other times, I have lost my temper and shouted at him, and spelt out the problems in his reasoning very bluntly, which of course causes an argument. but he often does reflect when the dust dies down and admit that he has been doomy and that his predicted outcome is one possibility among many....
what works well when I manage it is to do some things in an efficient and 'achieving' way (from booking the car in for an mot to dealing with relatives!) - but give him some tasks to do in the same vein along the lines of 'I've done this bit, now you do that bit'. And they are to be done promptly - no brooding and mulling and dragging feet while the storm clouds gather and he gets The Fear. I won't do everything for him. I'm actually very strict now about what. I will and won't do for him, tbh. At times in the past I have gone down the 'I'll just do everything myself' route rather than deal with tricky conversations with him - but a few years ago, I took a view of our relationship and realised it was just way, way too onesided - me doing so much was actually allowing him to carry on with all his doomy negative behaviour patterns unchanged and unchallenged. A key thing was insisting he learn to drive again, instead of having me drive him round. He didn't want to, he was very anxious about it - but for me, it was a deal breaker. If he couldn't take responsibility for that, he wasn't interested in me as a partner, but as a 'carer'. He learned to drive, and it is now a shared chore.
Like all these things, the more he does, the better he gets - more assertive in the right way, more able to go into situations without having overthought a pessimistic outcome etc.
I am very clear in my own mind that I am not a substitute therapist or counsellor. I cannot be. I have friends who have got caught up in some misguided romantic notion of 'helping' partners with quite major problems - and it takes over their life. that's just not me. It is not my job to make do a 'better' person, either. He is who he is, and ultimately, only he is responsible for himself. In a way, my perspective is very selfish, although not in a way I feel bad about! - as it is about being in a healthy relationship, which allows me and hm to achieve our potential.