Taylor Swift and Travis Kelce are the epitome of the ultimate modern-day fairytale ... but they aren't immune to challenges -- starting with insane schedules.

We're diving deep into a solid relationship that could hit speed bumps in "TMZ Investigates: Taylor & Travis: Ultimate Love Story," airing Monday on FOX at 9/8c on Fox.

Dr Phil kicks it off ... telling us the biggest threat to the pop idol and the NFL star is their jam-packed schedules ... not allowing them to spend enough quality time together.

'Millionaire Matchmaker' star Patti Stanger adds their long-distance romance is a challenge. She says the little things couples do together will be few and far between, with Taylor touring the world and Travis with his own peripatetic career.

There's also the obvious lack of privacy, which is often a major reason for Hollywood couples calling it quits. Taylor's been in the spotlight since forever, so she's kinda used to it. But Travis Kelce? He's already experienced the shocking lack of privacy since he began dating her.

"Shark Tank" star Mark Cuban raises the privacy point ... saying privacy for them is truly out the window.

"TMZ Investigates: Taylor & Travis: Ultimate Love Story" premieres Monday, February 26 9/8c on FOX and the next day on Hulu.