Doherty, 45, also goes into the reasons why she recently shaved her head as part of her ongoing battle against breast cancer, which she exclusively revealed to WHO last August.
"After my second treatment, my hair was really matted, like in dreadlocks. And I went to try and brush it out, and it just fell out," Doherty says in a preview clip from the episode. "I just remember holding onto huge clumps of my hair in my hands and just running to my mom crying, like, 'My hair, my hair, my hair, my hair.' "
"It was just shedding and it was driving me crazy," she says. "It was just clumps, and I was like, 'Just grab the kitchen scissors.' And my mom's like, 'Wait, wait, wait.' I'm just, like, 'Grab it.' She went and grabbed the kitchen scissors, and put it in a ponytail and she just chopped it off. And it was this cute little bob, but it wasn't enough, you know, it was falling out."
She then took the final steps and shaved her hair completely off.
"We did stages," she says. "We did a pixie. And then we did a mohawk, which was my favorite look. And then finally we had to get the shaver thing and just buzz it off."
Another heartbreaking stage for Doherty was getting sized for new bras following her mastectomy.
"It was traumatic and horrible, and I didn't think anything of it at the time, then my mom went with me and I broke down crying in the dressing room and ran out," she told ET this week. "And then sat in the car crying."
"People don't realize that cancer – yes, it ages you – but also, you can balloon up from the various meds," she adds. "There are so many different reactions you have and so I just – I didn't want someone to take a picture of me coming out of the grocery store and be like, 'Oh God, look at her.'"
If there is one thing Doherty is taking away from her ordeal, it's the fact that she can help others who are enduring cancer as well.
"I wanted to put it out there the way it felt the best for me to put it out there," she says. "And also, if I could help one person then it makes me go, 'Oh Okay.' It's easier to live with having cancer if I know I'm helping at least one person."
Doherty's three-part Entertainment Tonight interview begins on Monday (check local listings).
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