FML stands for f*** my life. It is also a website (fmylife.com) where people record horrible things that have happened to them. FMLers have really horrible F***ed up lives, and have probably recorded something on that website. Or they have said F*** my life several times.
Person #1: So I hear you got divorced.
Person #2: Yeah, she thought I was having an affair, so she talked to her mother about it, who hates me. Now my kids hate me, and I live in a crappy apartment. F*** my life!
Person 1#: Wow you are turning into a real fmler!