Why does Helen Keller say that the happy days of her life did not last for long?

Helen says that the happy days did not last because she became very ill and almost died, and was left blind and deaf.
The happy days that Keller speaks of are the early days of her childhood. She lived with her mother and father in a little house on her family land. As a toddler, little Helen was thrilled by the simplest things. Life was great. Then she became very ill. 

Then, in the dreary month of February, came the illness which closed my eyes and ears and plunged me into the unconsciousness of a new-born baby. They called it acute congestion of the stomach and brain. The doctor thought I could not live. (Ch. 1) 

The doctors did not know exactly what she had, so they did not know if she would survive it or what it would do to her.  She ended up both blind and deaf. This was a big blow at such a young age. Because she was barely walking or talking, she did not really have much communication and was plunged into a scary world of darkness.

I especially remember the tenderness with which my mother tried to soothe me in my waking hours of fret and pain, and the agony and bewilderment with which I awoke after a tossing half sleep, and turned my eyes, so dry and hot, to the wall, away from the once-loved light, which came to me dim and yet more dim each day. (Ch. 1) 

Since she was so young, losing her sight and hearing was very frightening. There was little her mother could do to help make her feel better or explain what was happening, because she was so sick. Poor Helen was in a great deal of pain in addition to losing more and more of her sight and hearing each day.