I cannot live on, relying on fragmented memories alone
For this she is telling him that if he ever wanted to leave her, she would be suffer to live without him because she can only relying on the memory to comfort herself.
Bu yao shi yao nuo yan zhi yao tian tian zai yi qi
Wo
If the day that you said you were leaving ever came
The sentence is a future continuous tense (progressive tense) which refer to the things where not yet happen but soon to be. Meaning she is afraid that one her love one will need to leave her.
In here, she is not really mean to loose the power of life. She's actually saying she rather to give up every things just to find someone that can be her soulmate.
qixi means essence but in another way to explain its mean habits or lifestyle. In this sentence, she is saying she willing to follow his foot steps or lifestyles.
Ye xu ren shi mou yi ren guo zhao ping fan de ri zi
Maybe I would just meet somebody and living an ordinary life. It's saying herself if she never met her love one, she will just meet somebody else and living an ordinary life with the person.
Ri zi guo de zen yao yang ren sheng shi fou yao zhen xi
How was my everyday life? Do I learn to cherish life? It's a question she asking herself if she didn't met her loved one, would she still want to cherish her life like now.
Ru guo mei you yu jian ni wo jiang hui shi zai na li
If I never met you, where will I be? It's a question where she asking herself, if she never meet her beloved one, where will she been? Maybe somewhere else without her love one.