Has Practicing the Law of Attraction Ever Backfired on You, Causing You to Lose Everything You Manifested and End Up in a Worse Spot Than Where You Started?
No. I do believe, though, that intention is important. Your intentions should be pure and not be powered by greed, arrogance, and ruthlessness.
Unlike some LoA disciples, I also believe that God plays a crucial role in manifestation and should be acknowledged, thanked, and—most importantly—listened to.
I don’t think it’s a coincidence that some “spiritual teachers” who were featured in the book/movie The Secret had such streaks of "bad luck." One of them went to jail for killing three people in a sweat lodge during one of his "spiritual warrior" retreats. Another, who was known as a greedy litigator, couldn't get their several-million-dollar house sold when they tried to.
Eventually, everyone gets what's coming to them... even if it's in the afterlife.
To manifest in accordance with God's will, you need to:
a) stay humble and don't give into hubris and pride;
b) not manifest “stuff” for stuff’s sake but only what's truly important and necessary for you;
c) think of other people and work on their behalf at least as much as on your own.
Manifestation is a dangerous weapon in the hands of someone who only cares about themselves… not only a danger to others, but to the immortal soul of that person him/herself.