“Teen Mom”: Gary and Kristina Shirley Enjoy Major Milestone with Leah as Amber Cries She 'Has No Choice' in Their Relationship

Amber Portwood laments the disconnect in her relationship with 16-year-old Leah

• Gary Shirley and wife Kristina enjoy close bonds with 16-year-old Leah
• Amber Portwood, Leah's mom, doesn't have the same bond with her daughter, who she's estranged from
• As milestones come up, Leah looks to Kristina to fill that role in her life

The co-parenting dynamic between Amber Portwood and Gary Shirley hasn't gotten any easier in the last 16 years.

The latest episode of Teen Mom: The Next Chapter shows how both of the exes are impacted by daughter Leah's estrangement from her mom. Gary and wife Kristina start the episode discussing how responsible Leah is after getting her first job without being asked to. Leah also runs track and keeps up straight As, making the couple proud. They decide that they want to get Leah her own car as a reward.

"Do we have Amber a part of that?" Kristina asks.

"F--- no," Gary replies. Of her current contact with himself and Leah, Gary says, "I've messaged Amber a few times. No reply."

"A person with bipolar disorder, I know they get up and down. And when she's up, she's feeling good. She might think everything's perfect. She'll want things to be a certain way, like, 'Oh, I really am sorry, but now I want you to instantly forgive me for years and years and years of abandonment.' It doesn't work that way," he continues.

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Related: Teen Mom's Gary Shirley Says Daughter Leah Wants His Wife to Adopt Her After Amber Portwood Refuses to See Her

"I want Leah to have a relationship with her mother for the sake of her, but at this point, it's really like... I don't feel like it's worth it."

Gary and Kristina do end up getting Leah's car and prepare to surprise her with it. The couple checks out the car and Kristina begins to get emotional about the milestone.

"It's gonna really hit when she pulls out for the first time from this driveway by herself. She is very well deserving of this. I mean, everything she's been through, the ups and downs her mom and... I think that this gift from you and I to show her that we love her. We're here for her."

Tearing up, Kristina adds, "It kind of makes me sad though because she's growing up. I've done all of her driving since preschool. Gosh, my mama heart."

Later, Gary and Kristina surprise Leah with the car. The teary-eyed teen hugs her dad, then Kristina, and tells her, "I love you." When she gets ready to take it for a ride for the first time, she says, "Mom, I want you with me."

In a confessional, Kristina explains, "For the longest time, Leah called me Kristina and I would say the last couple of years, it's been mom."

"There's always room for Amber and being a mom isn't just taking them to school, doing homework, taking them to appointments. It's being there every day, through all the troubles and the happy times and everything."

When producers show up to film with Amber, she refuses to answer the door. Then she asks one of the producers to bring her a pregnancy test. When they bring it to her and ask if she thinks she's pregnant, she shares, "It's possible."

"It was an ex. It was a complete after shock breakup situation. I'm just like freaked out right now. I'm scared. Just pray for me, that I wasn't too much of a jackass," she says.

As they come back, Amber admits she never took the test and isn't pregnant. She used it as an excuse to get out of filming.

"I'm sorry. There's been many times in my life where I have to lie to get out of doing things because I'm not able to get ready, get dressed, move around, do anything. You just lie to people... You want to make up something to where they're like, 'Okay, you don't have to do anything.' When you start lying to be alone? That's me at my darkest."

Touching on the trauma from her brief relationship with Gary Wayt, Amber continues, "When I think about Leah and James, I don't know what's going to happen. I fought for my son for four years and I still might have lost him, but I didn't lose him because I kept fighting after he moved to California and I kept fighting for my relationship with him and to this day, I have one."

Turning to her relationship with Leah, she continues, "With my daughter, she doesn't want to be around me. I'm understanding right now that I don't have a choice."

Moving forward, Amber says she's "Trying to figure out how I'm going to wait for my daughter and not be a horrifically depressed person. I've never been a mom that didn't care, just a f---ed up mom."

Catch new episodes of Teen Mom: The Next Chapter on Thursdays at 8 p.m. ET on MTV.