You did not fail the diet. The hunger came back louder.
After a hard diet, hunger hormones can stay high for a year, which people often call failure.
Fast loss on a crash diet is not the same as fat leaving.
Fast drops on a crash diet often come from water and stored carbohydrate, so the scale can climb again when food returns.
The diet worked for six weeks. Then every pound came back.
Crash diets can drop the scale for a few weeks, then hunger and a slower burn help the weight return.