我会珍惜 我们所有的回忆

I "ll cherish all the memories about us .（all our memories ）

因为 那些 和你在一起的日子

cause the days with you

是我一生最幸福的时光

is the happinest time in my life.

或者后两句翻得生动一些还可以译成：The hours I spent with you were happiest of my life.(用hour显得再一起的时间更令人珍惜)

整句话就是

I "ll cherish all our memories .Cause the hours I spent with you were happiest of my life.

希望可以帮到楼主~。