How to be happy with a sick baby?

My daughter was born on Nov 23 at 26 weeks gestation weighing only 1lb 10oz. Needless to say she was immediately flown to Winnie Palmer hospital and has been in the Nicu ever since. She is improving wonderful but I still have guilt at not being able to carry her her to full term, and all the pain she has been through. And ever time I smile or feel happy, every time I hold and care for my other daughter, or take her on outings. I feel so bad. Like how can I feel happiness when my baby is sick. I see her as much as I can and call to check on her all the time but I cant shake this feeling that I'm a bad mom. How do I feel less guilty?