The two starred on the MTV reality soap, which ended in July. They separated last May after one year of marriage.
Montag cited irreconcilable differences when she filed for separation from Pratt in June. Pratt said he would release a sex tape featuring Montag.
The 27-year-old Pratt tells The Associated Press in an e-mail sent Wednesday the demise of their relationship was so painful he wanted to lash out. He admits making "certain private issues public" and "horribly embarrassing the one person who meant the world to me." He says his behavior was "outrageous and infantile."
Pratt now says he's failed at handling his divorce with dignity and maturity and vows to work on himself.