Bristol Palin trashes the father of her 2½-year-old son Tripp, calling Levi Johnston 'the Gnat' in her new memoir 'Not Afraid of Life: My Journey So Far.'

The 'Dancing With the Stars' finalist reveals she lost her virginity to Johnston when she got drunk for the first time on wine during a camping trip, then woke up in a tent with no memory of the blackout sex. Though she and Johnston agreed they should wait until marriage, she admits they were soon intimate again. She claims that in their rocky relationship, Johnston "cheated on me about as frequently as he sharpened his hockey skates."

Palin's pregnancy at age 17 was revealed to the world when her mother, Sarah Palin, was chosen as John McCain's vice-presidential running mate in the 2008 election.

Bristol's new book, co-written with Nancy French, goes on sale this week, and features the young mom nuzzling Tripp (obscuring part of her own face) in the cover photo.