"I was frustrated. I was really, really torn," Katie said during a Wednesday appearance on Married at First Sight: Unfiltered, which is hosted by Season 1 MAFS participant Jamie Otis.
Katie therefore admittedly had "cold feet" and was grappling with the decision on whether to actually go through with the wedding.
While her ex had "messed with [her] head" and failed to commit when they first got together, Katie had no idea whether she'd feel a connection with the groom she had been matched to marry by Married at First Sight experts Pastor Calvin Roberson, Dr. Pepper Schwartz and Dr. Viviana Coles.
When asked to explain how she felt when her ex came back into the picture, Katie said on Unfiltered, "I knew how I felt about him. That was something that I was sure about."
"So it was really hard for me to make a decision going into this -- not knowing how I was going to feel," she said.
Jamie then asked Pastor Cal, who also appeared on the Unfiltered episode, whether exes surface frequently when singles choose to appear on Married at First Sight.
"Yes! They come out of the woodwork because they want to make sure they still have control in the situation," Pastor Cal explained.
"They don't want you -- but they don't want anyone to have you either," Jamie pointed out.
"Exactly," Pastor Cal noted.
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"It's just a reaction," Pastor Cal reasoned. "It's nothing real."
Jamie revealed she had "a bit of an ex situation as well," going into Married at First Sight's first season, when she married Doug Hehner. (Jamie and Doug are still happily married and expecting their second child).
"You have feelings for them and you wonder -- it's like you're at a turning point, a crossroads in your life: Which way do I go, and is that the right way?" Jamie shared.
"It's a leap of faith and it's a huge risk to marry a stranger. But it can work out, and it turned out to be the better decision for me. So I hope it does for you to."
Both Katie and Derek were all smiles and said "wow" when they first saw each other. Derek told his future wife that she looked beautiful, and Katie thought Derek was "so cute" as well as "tall, dark and handsome."
The couple kissed many times throughout the night and both felt comfortable in each other's arms while slow dancing. Derek, a 26-year-old cyber security engineer from Maryland, called being with his wife "a really cool feeling."
Viewers, however, have yet to see Katie and Derek's first night together as man and wife.
When asked to reveal what it would take for Katie to want to consummate her marriage with Derek on their wedding night, Katie replied, "I [tried] to go into it without any expectations."
Katie did, however, say before her wedding day that she'd be open-minded about sleeping with her groom.
"If we are really into each other, then whatever happens, happens," Katie, a 25-year-old mental health professional from Woodbridge, VA, disclosed on Unfiltered.
"Interesting coming from the bride who's not even sure if she's going to walk down the aisle!" Jamie joked.