Oh bless you. You've been through the mill but how you feel is a normal response to trauma. I have been through similar and, like you, I 'crashed' a bit after the experience was over.
The potted version of our story is that DD was conceived via donor egg (premature menopause - a traumatic diagnosis in itself), I lost her twin at 5 weeks but we didn't know until 7 + 2 that I hadn't completely miscarried. At the 12 week scan they couldn't get a very good picture - slightly low fluid and (I didn't find out until much later) DD was measuring small. 20 week scan DD was measuring 3 + weeks behind- we were rushed to the fetal medicine unit, fortnightly scans, refused tests, 'hung on in there' (DH also lost his job around this time).
At 27 weeks I started to feel pretty rough- V swollen, giddy etc, 29 weeks admitted to hospital with pre-eclampsia, somehow managed to hold on until 35 weeks (at 32 weeks we were given a misdiagnosis during a scan and my consultant started talking about a termination or inducing and not resuscitating the baby. . . ) second opinion turned everything back on it's head.
Eventually, DD was born at 35 weeks weighing a teeny 2 lb 10 ozs but perfect. 28 days in SCBU, 2 weeks in hospital she was born in then 2 weeks at more local hospital.
For the first three months I was on the biggest high. I had my beautiful, tiny little girl against many odds. But then at 3 months she developed a hernia, then they were worried about an odd marking at the base of her spine, she had a hernia repair. . . I did an awful lot of 'googling' and convinced myself that DD had some awful, rare conditions (she had some genetic testing after she was born as the growth restriction can be a sign of chromosomal conditions). I slowly started to feel as if I was losing my mind. I was unable to sleep for fear of something happening to DD. This was where it was almost a good thing DH was out of work as he took DD through to the living room every morning when I'd been up all night feeding her and googling. I felt panicky and was having intrusive thoughts and found sleeping once the sun had risen safer as I knew DH was awake and watching over DD.
As we'd had complications and DD wasn't gaining weight that quickly, we still had weekly visits from the loveliest community special care nurse and when I broke down during one visit, she realised that I was actually a bit of a mess. She arranged for me to see a counsellor based on the special care unit and that was just what I needed. I felt I could actually voice my biggest fears and discuss my intrusive thoughts without upsetting my DH or anyone close to me.
Whilst I've had some amazing moments over the past years, it's been difficult to shake my sadness at having a far from normal conception, pregnancy, delivery and start to family life. I've felt cheated, angry and for the longest time felt unable to be relaxed enough to allow myself to feel happy as I realised that bad things can happen. I didn't end up going to my GP as I'd had counselling but I have had moments (and still do sometimes) where I think I should see my GP. We have other stuff going on in our lives too though.
Sorry to have waffled on but I sometimes thinking sharing experiences can help. To answer your question, things that helped me cope as well as the counselling were: trying to remember that 'a thought is just a thought', I used Bach's Rescue Remedy a lot, I talked about my experiences quite freely as voicing things made them feel less scary, I considered going to the coffee mornings run for parents who'd had babies in SCBU but never got round to it- these might help you if available and I've tried to be kind to myself. It's awful having a difficult pregnancy and being so scared of how well or not your baby will be at birth then not having them with you when they're in hospital. Then I should imagine the episodes your DD has had with breathing issues etc must have been so scary. When you're in the middle of all of that, you cope because you have to. You deal with it and adrenalin gets you through. It's when the trauma is over that reality hits you and then you crumple. Totally normal, I'd say!
Anyway. I need to go to sleep now but I'm thinking of you. You will get through this.