She added: "Sadness is the only thing that's ever made me lose weight.
"Two years ago, during the last gasps of my six-year relationship, I lost weight.
"Not a little weight. Not the kind of weight where your bras feel kind of generous and you marvel at your subtle but oh-wow-it's-definitely-there-now clavicle. No, it was a lot of weight."
The 33-year-old actress found it bizarre that she didn't pile on the pounds like most women do after a breakup.
She said: "At the end of that relationship, the weight was falling off in double digits but, as I explained to my closest friends, I experienced none of the heady triumph of the women showing off their formerly-huge jeans in a full page ad for weight-loss pills."
The writer also revealed that she "very quickly" found a new "lover" after their split.
She recalled: "When my boyfriend and I broke up, I very quickly took a lover.
"He was someone I had known from childhood and he was, in many ways, exactly as I remembered him: goofy and physical, boyish and sweet. I went from being touched barely at all to being the very definition of touch."
Lena also did a stint in rehab and once she was weaned off drugs she started to feel "real hungry".
She said: "It took a week, but as the drugs left my body and I made peace with the fact that I was here to heal, not just to cut my own bangs and weep (just kidding: you can't cut your own bangs because they won't give you scissors). I got real hungry, real fast. If it wasn't nailed down, I ate it."
The 'Once Upon a Time In Hollywood' star says meat pies are her "ecstasy" and the only thing that can "comfort" her other than her mum and she claims the savoury treat keeps her "alive".
She spilled: "After one bite I am in ecstasy.
"After the second, I miss my mum, the only person or thing that can rival this pie for comfort.
"After the third I shed a tear. For all the bad bites I've ever taken. For all the things I've ever hated and finished anyway and all the delicious things I haven't allowed myself to finish. Loving this meat pie means I'm alive."