The 26-year-old reality star referred to the two-time Olympic gymnast as his "first love" while recalling his "worst heartbreak" for hosts Keltie Knight, Jac Vanek and Becca Tobin.
"While it was confusing and it was hard in the beginning, it also taught me a lot about myself -- what I need, what I want, what I can do better in a relationship," the star said of his split from Raisman.
"It was a big step for me. It was really tough," he added.
Underwood split from Raisman in June 2017 after several months of dating. He voiced his love and respect for the athlete in an interview with Us Weekly in July during her involvement in Larry Nassar's case and sentencing.
"To be very candid about it, she's going through enough right now," the star explained of keeping his past relationship with Raisman private. "She doesn't deserve to be in this at all."
"I have so much love and respect for Aly and the voice that she is and the role model that she is for every young woman out there," he added.
Underwood competed for Becca Kufrin's affections in The Bachelorette Season 14, which aired over the summer. He is now looking for love in The Bachelor Season 23, which premiered last week.