Forrest was born June 12, 1998. He grew up in Chipley, Florida and attended Chipley High School. Forrest scored over 3,000 points during his high school career and led the team to a state title as a senior. Forrest also played in AAU with the Alabama Challenge and the Georgia Stars, helping him secure a top-50 rank in his high school class. For college, he signed with Florida State, turning down offers from Miami (Florida), Georgia Tech, Tennessee, UCF and Wichita State. As a senior at FSU, he was named to the ACC All-Second Team, the ACC All-Defensive Team and the Academic All-American Third Team. Afterward, he declared for the 2020 NBA Draft. Forrest played four years at Florida State. He was a reserve during his freshman and sophomore campaigns. As a freshman, he averaged 4.9 points, 2.7 rebounds, 1.6 assists and 1.2 steals. As a sophomore, he averaged 7.9 points, 4.9 rebounds, 4.1 assists and 1.6 steals. Forrest became a starter as a junior in 2018-19, averaging 9.3 points, 4.5 rebounds, 3.7 assists and 1.9 steals in 29.9 minutes. He made improvements again as a senior, averaging 11.6 points, 4.4 rebounds, 4.0 assists and 1.9 steals in 30.8 minutes. Forrest's excellent senior campaign led to him being named ACC All-Second Team, ACC All-Defensive Team and Academic All-American Third Team.