Once I try to understanding（要用原型） you and accept your love.I just don't wanna hurt you.I （增加am）afraid one day I will leave you .It will be a great strick(打击的单词不是这个,应该是
STRIKE) to you.So,(不需要逗号)I decide to leave you now.But your behavior made me so unfirm （没有这个单词,你可以使用confused疑惑,Uncertain不确定的）with my decision.
即可以更改为：
Once I try to understand you and accept your love.I just don't wanna hurt you.I‘m afraid one day I will leave you .It will be a great strike to you.So I decide to leave you now.But your behavior made me so uncertain with my decision.
补充：请三思再决定分开,不要错过每一个爱你的人和你爱的人.