Cheryl would tell her not to worry,it didn’t matter
Cheryl告诉她（Cheryl的妈妈）不要担心,没什么事.
still her mother felt she had let her down and apologized at each good-bye
然而她妈妈仍然觉得她（妈妈）对不起她（Cheryl）,而且每次相互道别的时候都要（对Cheryl）道歉.
until the day Cheryl found a way to give her mother the same reassurance ( her mother had given to her so many times 省略的that的定语从句,修饰reassurance)
直到有一天Cheryl发现了一个让她妈妈放心的方法（这种放心的感觉是以前一直以来妈妈不断给与她的.）
我这个理解对吗?至于标点符号.still 前面用逗号比较好,意思比较连贯.