John Broadhurst, a property developer, is said to have "totally lost it" during "drug-fuelled" intercourse.
John Broadhurst, 40, was previously cleared of murdering Natalie Connolly and causing her grievous bodily harm on the directions of the judge following legal submissions part-way through his trial.
The property developer, said during his trial to be worth up to £15m, admitted manslaughter by gross negligence after leaving his girlfriend to die at the foot of a staircase.
Jailing Broadhurst, Mr Justice Julian Knowles accepted that the couple had been happy together during their brief relationship.
But the judge told him: "You were capable of taking decisions and making choices. This was grossly irresponsible behaviour by you.
"You left that vulnerable young woman to die in the saddest and most avoidable of circumstances. You showed blatant disregard for a very drunk and injured woman."
Prosecutors had alleged that Broadhurst "totally lost it" during drink and drug-fuelled intercourse at their home in the village of Kinver in Staffordshire.
The trial heard that the couple had "rough sex" on the night of Miss Connolly's death.
It was also told Broadhurst dialled 999, informing the operator he had found his partner "dead as a doughnut" at the bottom of the stairs.
The 26-year-old had suffered more than 40 injuries, which a post-mortem later showed included serious internal trauma.
The mother-of-one was pronounced dead at the scene by a paramedic on the morning of 8 December 2016.
A trial at Birmingham Crown Court was told that Broadhurst claimed Ms Connolly was injured as a result of consensual sexual activity after both of them drank alcohol and took drugs.