You're comparing your middle to their beginning's end. Everyone's story looks effortless once they've cut the years you're living now.

The thought behind it

This line takes the other person out of the frame, because the comparison was never a fair one to begin with. The Honest Mirror trades reassurance for accuracy, on the theory that accuracy is the kinder of the two. Everything hinges on “middle”, which is the part a politer version of this sentence would have left out.