Sharpe then moved around different car parks, forced her to drink alcohol and sexually assaulted her.
Police carried out extensive CCTV inquiries after the girl was reported missing, which led to Sharpe’s car being identified.
He was later arrested in a hotel car park in Knutsford, Cheshire, in the early hours of the following day.
In a police interview, Sharpe claimed he had been feeling bad so he kidnapped the girl thinking it would be easier than kidnapping an adult.
He said he was not thinking clearly and his intention was to release her when it was light, then take his own life, the CPS said.
He also claimed his motives were not sexual.
CPS spokeswoman Hannah Gee said: “In this terrifying incident the victim was held captive for eight hours while Sharpe forced her to drink alcohol and sexually assaulted her.
“Sharpe gave no thought to the impact his actions, for his own depraved reasons, would have on his victim.
“Although his guilty pleas today have spared her from the ordeal of a trial, I would like to thank the girl and her family for supporting this prosecution and ensuring this dangerous offender is back where he belongs.”