Fossesholm, who competes for the Eiker SK, became Norwegian junior champion in 2018 in sprint, over 7.5 km classic and over 5 km freestyle. The following year she won the silver medal over 5 km freestyle and 15 km mass start race and the gold medal with the relay at the Nordic Junior World Ski Championships in Lahti and was again Norwegian junior champion in sprint, over 10 km freestyle and 5 km classic. After two third places over 10 km at the FIS races in Beitostølen at the beginning of the 2019/20 season, she took part in the cross-country skiing world cup in Lillehammer for the first time in December 2019 . With the 18th place in the skiathlon she got her first World Cup points. At the Nordic Junior World Ski Championships 2020 in Oberwiesenthal , she won the gold medal in the classic over 5 km and in the 15 km mass start race. She was also sixth in the relay and fifth in the sprint.