"I love you very much, my darling! If it lasts a lifetime, then, we will be together as long as we live/during our lifetime. We made a promise, so we will abide by/keep our promise till the end. I love you."
The part "If it lasts a lifetime, then, we will be together as long as we live/during our lifetime." in the original seems a bit strange. What is meant with it isn't clear. But, my offer may help you more or less.
If there is anything that is not clear with my bridge, please ask me.
Hey Handyy User10 transalated this part as:
I love you very much my love. We will be together for a lifetime, as long as this will last. We gave (our) word and will keep it till the end. I love you.