"My mind is switching every night," he said.
Riley's struggle offers a window into the workings of the recruiting battle between a Trojans staff installed in the midst of one of the most tumultuous times in USC football history and a UCLA staff that senses opportunity.
USC Coach Lane Kiffin had targeted Riley, a lifelong Trojans fan, even before Kiffin left Tennessee to return as Pete Carroll's successor.
And UCLA Coach Rick Neuheisel had zeroed in on Riley long before Carroll's mid-January departure for the Seattle Seahawks.
NCAA rules forbid coaches to speak publicly about high school players before they sign letters of intent, but Allen Wallace, national recruiting editor for Scout.com, described the L.A. battle for Riley as "kind of like a grudge-match mentality."
A UCLA victory, he said, would buoy Neuheisel and his staff.
"If they would be able to rip Dietrich Riley away, it just reinforces their position that the decade of dominance is over," Wallace said. "They can say, 'We can fight you on the recruiting front.' "
Crenshaw linebacker Hayes Pullard and Ventura St. Bonaventure lineman Giovanni Di Poalo are among other recruits who have yet to decide between the Bruins and the Trojans.
The 6-foot-1, 205-pound Riley has received more than 30 scholarship offers -- the first coming during his sophomore year from Carroll after Trojans coaches watched him perform at a USC football camp he had attended annually since age 12.
"I guess the news broke out," Riley recalled during an interview last week at St. Francis. "That's when the competition began."
Two weeks after USC offered, UCLA did the same.
Thus started two years of wooing, former USC defensive coordinator Rocky Seto the point man for the Trojans, Neuheisel leading the Bruins' push.
Riley made a "silent," or unannounced, commitment to the Trojans last December, telling Carroll he would be on campus next summer.
"It was on the down-low because I wanted to make it a surprise on signing day," he said.
But Riley also called Neuheisel to inform him of his intent.
"He was completely fine with it," Riley said. "But he just said, 'I actually love it now that you're a commit because I can actually come after you harder.' "
Neuheisel got an unexpected assist a few weeks ago when news broke that Carroll was close to returning to the NFL after nine seasons at USC, a run that produced two national titles and seven consecutive Bowl Championship Series bowl appearances.
Riley was making an official weekend visit to UCLA when the Carroll reports surfaced. He said his phone began "blowing up" with calls from friends asking what he was going to do.
"I just couldn't believe it," he said.
Riley and other recruits attempted to reach USC assistants, but their calls, for the most part, were not immediately returned. Riley finally heard from the Trojans the following Monday.
"That hurt," Riley said of Carroll's departure.
Like many recruits who had publicly committed to the Trojans, Riley reconsidered his private commitment.
USC suffered another blow when Kiffin decided not to retain Seto.
"That was kind of a heartbreaker," Riley said.
Riley, who made official visits to Tennessee, Notre Dame, LSU and UCLA, has not ruled out playing for the Trojans. He even canceled an official visit to Mississippi last weekend so he could make one to USC.
It was his second go-round with a Kiffin staff, his trip to Knoxville having left him feeling unwanted.
"Even though I was told that I was the No. 1 safety prospect for that school, I just didn't feel it when I went out there," he said.
Riley said Monday that his USC visit, which included several other top recruits, went well.
So will it be the Trojans or the Bruins?
Both coaching staffs have told Riley that he could play right away.
New defensive coordinator Monte Kiffin showed him where he could fit into USC's scheme. UCLA offensive coordinator Norm Chow told him he might play both ways, running plays out of the wildcat formation.
It all sounds great to Riley, but he acknowledges both programs' downsides.
UCLA has not played in a major bowl game in years. USC could incur sanctions from an NCAA investigation that has already prompted the school to self-impose penalties against its basketball program.
"That's worrying me a bit because I don't want to get there and be banned from a bowl game one or two years," he said. "You see what happened to the basketball team and that's kind of nerve-racking."
So, apparently, is choosing between playing at the Rose Bowl or the Coliseum.
"With UCLA . . . this is my city," said Riley, a Pasadena resident. "So my family won't have to drive so far, they can tailgate right there.
"And then with USC I'm at the Coliseum, such a beautiful atmosphere with so many great players who have played before me on that field."
Riley has friends at both schools, including receiver Randall Carroll, tight end Morrell Pressley and quarterback Richard Brehaut at UCLA and defensive back Torin Harris at USC.
All have told him they would love to be his teammate, but to make the choice that's best for him.
The coaching staffs have done the same.
"That's what I love about both schools -- they don't talk negative about one another," Riley said. "They're not trying to down-talk another school to make me more attracted to them."
That only makes the choice tougher.
"Every day," he said, "my mind is changing."