一辈子:forever
爱你一辈子:I love you forever.
I will love you with all my life for ever!
I promise to love you for the rest of my life.
I will always love you.
有一句诗也可以表达这个意思,很有美感很有意境的来着:
Robert Browning的 Grow old along with me, the best is yet to be.
I love you all my life.