The British Heart Foundation have a Heart Helpline on 0300 330 3311 and they should be able to answer any queries your dh has about his ongoing pain and its cause - which is, of course, the invasive nature of the surgery he underwent and the extent to which his sternum and ribs in particular had to be brutally misplaced to gain access to his heart.
Having broken a couple of ribs in the past, I know the feeling of every movement being painful; every cough or deep breath feeling as if it would cause my ribcage to come apart or cave in. Lying down was a no-go area and only sitting in a semi-reclining position and keeping perfectly still afforded any relief until weeks later, seemingly miraculously, the pain disappeared virtually overnight.
Obviously, a couple of broken ribs can't be compared to the punishment your dh's body has sustained and it will take some time for his sternum to fuse back together - at the moment it's held together by wires which may account for the 'pulling' sensation you've said he experiences.
Has he been seen, or is he being visited, by Imperial's Cardio Rehab Team? What pain relief has been prescribed for him and are district or other nurses cleaning/checking his scars on a regular basis and generally monitoring his progress on a regular basis?
I have no doubt that the nurse who caused your dh to need an additional operation to insert a line did so accidentally, but nevertheless it was an act of incompetence at best and negligence at worst and your dh must have felt utterly despairing at the prospect of yet another op so soon after his ordeal.
God knows what it took him to get through those weeks of girding himself to undergo life threatening surgery only to be told on 2 occasions 'the op's off' and then to have to put himself in the hands of a cardiac team that the Clinical Director described as incompetent.
Your dh is a man of rare courage but to be bought so low by an inattentive nurse as to get himself home just a few days after major surgery which, not so long ago, would have seen him in hospital for 3 months and a lengthy stay in a convalescent home, is indicative of the sense of utter despair and hopelessness he was feeling - that must have been the absolute last straw for him, as it would have been for so many.
It's probable that the sense of confusion you reported here when he came round from that totally avoidable op has also played a part in depressing his customary optimism, as that must have been yet another frightening experience as he came to the realisation that part of his memory had vanished, albeit temporarily - given everything he's been through, that alone may have served to have tip him into an altered state, as it were and the anesthetics that he was given are known to have a depressing effect which can last for some weeks/months.
On the plus side, your dh survived a 10-12 hour op which appears to have had a successful outcome, he's out of the hellhole of hospital, he has been safely restored to his home without having acquired any of the infections that are rampant in many NHS wards, and he is presumably having full dialysis in the secure environment of his home.
If he's not getting any other nursing care, raise merry hell. Get Imperial's Cardio Rehab Team off their arses round to monitor him and give him the reassurance he needs that it will get better and that, as it gets better, he will get better.
There are ways of delivering 'positive psychological input' without him having to unburden himself of a shrink's couch. Bring autumn into the house and place a few fresh flowers beside him, a couple of acorns and conkers, some amazing leaves. Play Mozart (well known for stimulating the feelgood factor), buy/beg/steal/borrow a daylight therapy (SAD) light box, and do what you're no doubt an expert at doing - being gentle with him while not babying him.
There's a long road ahead, JandJ, but you're out of the tunnel. Apart from sparkles and cocktails and delectabe foodstuffs, we've got a limitless supply of tissues and, if you don't mind some of us sobbing alongside you getting the occasional bit of wadding from the quilt in our eyes, we'll continue to hold hands and keep your circle unbroken. Whinge, rail, moan, rage and quietly, or loudly, despair all you want, honey. That's why we've gathered here and we'll be your steadfast fan club companions throughout.
Btw, any rumours of me flying high in a tincan the other day are entirely false. I was of course sitting at the rear of a certain narrowboat, watching the ripples left in its wake, and encouraging Ringo and his springer pal to chase rabbits and rub noses with any equines that happened to be passing - thanks to sw it was a thoroughly memorable day.