Three years after giving up their daughter Carly for adoption, Teen Mom stars Catelynn Lowell and Tyler Baltierra still struggle with their decision, but tell Us Weekly it does get easier as time goes on.

"Adoption is a never-ending process," Lowell, 19, admitted to Us during a recent interview promoting Tuesday's Teen Mom season premiere. "Some things have gotten easier -- our relationship with [Carly's adoptive parents] Brandon and Theresa has gotten stronger over the years -- but we still have things that we work on."

Though their daughter just turned three, the couple will chronicle their second annual visit with Carly -- which coincided with her second birthday -- on Tuesday's episode. "It's one of the most emotional things on the premiere," Baltierra, 19, explains. "She's totally different [now] -- she's walking and talking!"

Lowell says that Carly -- who recently became a big sister when Brandon and Theresa adopted a little boy -- is already taking after her biological dad. "She has the biggest attitude now, like Tyler. She's so headstrong and confident. It's funny," Lowell says. "She's really good at counting and knowing all her colors. She started preschool and she's really doing great in school."

For much more from Lowell and Baltierra -- including if their daughter will attend their upcoming 2013 wedding -- watch our exclusive interview now before Teen Mom premieres the couple's final season Tuesday at 10 p.m. (EST) on MTV.