I agree with FATE, sorry, she's gone from having you come to her every time she cries and be given breastfeed for comfort for her whole life and now she's just left in her cot crying for 45 minutes-that's a long time.
She won't understand and will be desperately hoping you will comfort her and then give up.
Sorry, I know that's not what you want to hear and I know that lots of people say CC works but like FATE says, that's a huge change for a little baby.
There is evidence that sleep training (various methods) does work in the short term but not in the long term.
None of the studies that have been done have found an effect lasting 6 months or more after the sleep training methods was originally used, and found to 'improve' babies sleep. This means that the initial improvement, or 'benefit' of using the method disappeared, and outcomes were the same for the group of babies who had been 'sleep trained' as for those in the control group who had not
www.isisonline.org.uk/how_babies_sleep/sleep_training/research_evidence/
Taken from ISIS
A recent study ( Middlemiss et al ) demonstrated that mothers and babies undergoing a controlled-crying intervention started out with matching, synchronous, hormonal stress responses (babies cried at bed-time, their stress hormone levels increased, and their mothers' stress hormone levels also increased). After three days, babies had ceased to cry at night, and mothers' stress hormone levels dropped, however babies levels - despite the fact they were no longer crying - remained high. This suggests that the babies behaviour had adapted to being alone for sleep, but that their physiology had not. The response of the babies in this study lends support to the theory that babies who undergo sleep training via extinction may be learning to 'give up' rather than to 'settle' -- outwardly the two behaviours appear the same, but inwardly the babies physiology is very different. As well as being physically separated from their mothers, the sleep trained babies were no longer in physical synchrony with their mothers as their mutual stress response link (maintained by infant crying) had been broken.
I know it's a very emotive subject and one which people have strong opinions on. I do understand you're in a difficult and stressful situation of having to go into hospital.