The 39-year-old television personality discussed her issues with Kim in an interview with E! News after calling the 37-year-old reality star an "evil human being" in a Keeping Up with the Kardashians promo that debuted Thursday.
"I think it started at the end of Season 14, a lot of tension building between us," Kourtney said. "I felt like every day almost I was being picked on and criticized and I just couldn't take it any more."
Kourtney said she's still working out her problems with Kim and their sister Khloe Kardashian.
"Honestly, I think it's ongoing," the star said. "I've really been working on myself a lot and I think through my growth, I'm better at expressing myself. I think in the past I've always been great at like not being emotional. I think that I'm better now at expressing my feelings."
"It's a lot of communication instead of letting things go back into the way that they were. It's definitely a process and you'll see it throughout the season," she added.
The promo showed Khloe getting a phone call from a crying Kourtney after the mom-of-three clashed with Kim over the schedule for a photoshoot. Kim told Kourtney she is "the least exciting to look at."
"Khloe, you don't say things like that," Kourtney said of Kim's remark. "You guys just have really different values than me. I choose to be a mother to my three kids. I'm not here looking for another job. I already work enough, more than I would like to."
Kourtney's call with Khloe was interrupted by Kim, who went off on her sister about their scheduling conflicts.
"You're a very distraught, evil human being. I don't want to see you, okay? I don't agree with who you are as a human being," Kourtney told Kim. "I'm not going to live my life around people that make me feel not good on a daily basis."
Keeping Up with the Kardashians Season 15 premieres Sunday, Aug. 5.