What line sums up the nature of J. Alfred Prufrock's past life?
(Note: I'm not sure what is meant by "past life," so this answer will deal with Prufrock's nature, which is probably what is being asked.)
The line that sums up J. Alfred Prufrock's nature is "There will be time, there will be time..."
The reason this sums up Prufrock is simply that he's a character who has doubts about himself and always postpones what must be done because he assumes there will be time to do it later.
The poem begins with Prufrock, the speaker, asking someone, himself perhaps, to go for a walk. He has an "overwhelming question" to ask. But he is a character full of doubt as he asks himself, "Do I dare?" numerous times. And his excuse for doubting himself is that he believes that "there will be time."
The time motif runs throughout the poem, as Prufrock imagines himself old, balding, with decaying teeth ("Do I dare eat a peach?") wearing "the bottoms of my trousers rolled."
Prufrock discovers that time is the villain when it comes to delay. He knows that if he delays asking his important question too much, he'll miss his opportunity and "grow old."