Xiao Fang wondered why such a strong woman was able to leave and abandon her all alone while deciding to run away. However, she could accept and even wholeheartedly bring Yao Ling up all alone... At this point, she really envied Yao Ling ah~! From the story, she could even feel Jiu Lan's love for Yao Ling.
Xiao Fang tried to erase that thought away because she knew how hard Yao Ling would be if her mother didn't pick her up at that time --- she most probably would be dead. At least, Xiao Fang had a loving family and she never once had to do hard work. If she was still unsatisfied with this, it would show how greedy she was.
Thus, she was able to calm her mind down.
Jiu Lan did that actually because she also felt guilty towards her real daughter --- that was the reason why she poured her love solely to Yao Ling. When she ran away from the Han Kingdom, her mind was still full of depression and she didn't feel guilty for leaving Xiao Fang behind.