Ceon Broughton, of Enfield, north London, was found guilty of being responsible for the death of Louella Fletcher-Michie after he "bumped up" her dose of the hallucinogenic drug 2-CP.
The 30-year-old was also found guilty at Winchester Crown Court on Thursday of supplying Miss Fletcher-Michie with the drug before her death at the Bestival music festival in September 2017.
He had previously pleaded guilty to two other charges of supplying drugs to Miss Fletcher-Michie at Glastonbury festival in June 2017.
And he was in breach of a suspended prison sentence for two charges of possessing a lock knife and a Stanley knife blade.
Miss Fletcher-Michie's father, mother, Carol, and sister, Daisy, read emotional victim impact statements to the court.
Mr Michie told the court: "I wake up every morning to face life starting again without Louella, our daughter, our sister, our friend, our family now broken.
"For what? It makes no sense, our beautiful Louella should still be with us on any measure of humanity.
"No more yoga with my daughter, no more running round the outside of the Arsenal stadium with my daughter, nor more family fun with my daughter, her life cut cruelly short."
Daisy Fletcher-Michie said that she had been diagnosed as having post-traumatic stress disorder and had suffered panic attacks following her sister's death.
She said: "Our lives have been torn apart by the actions of someone who was supposed to love Louella, the most-loved person I have known."
Carol Fletcher-Michie, a former singer with Hot Gossip, said: "Having to learn to live with this for the rest of our lives, it's thrown everything I believe in life into question."
She added of the defendant: "We do not think Ceon is evil, he was stupid, massively selfish and he lied."
Sentencing Broughton, the judge Mr Justice Goose told him: "You were only concerned for yourself. You didn't want to be arrested, you were more in fear of that than in getting help."
The prosecution claimed during the trial that Broughton had failed to take "reasonable" steps to seek medical help for Miss Fletcher-Michie.
The jury was shown videos taken by the defendant as her condition worsened - and possibly after she had died - after she had taken a "large dose" of the psychedelic drug on the afternoon of September 10.
In clips shown to the court, Miss Fletcher-Michie repeatedly shouts at Broughton to telephone her mother, Carol Fletcher-Michie, but he tells her to "put your phone away".
Her mother eventually contacted Broughton at 6.48pm and heard her daughter "screeching" before she rushed with her husband to the festival site in a bid to find her daughter.
Prosecutor William Mousley QC said Broughton did not get help because he was handed a suspended jail term one month earlier and feared the consequences.
Stephen Kamlish QC read a statement to the court from Broughton saying: "Sorry I didn't do more to save Louella, sorry for the suffering I caused to everyone who loved Louella, I want to make things right."
By Richard Woodward