Liam Hanley, 32, told jurors that when he entered the bathroom of the Sussex Wharf flat he found Sarah Harries standing naked holding a knife.
Hanley, unemployed and of no fixed address, is charged with attempted murder in connection with the incident on September 4, 2018.
Miss Harries suffered up to 39 stab wounds including having her throat slit, according to the prosecution.
This afternoon Hanley stepped into the witness box at Lewes Crown Court to give evidence.
Answering questions from his defence barrister Rebecca Upton, he told the court how his father was violent and he left home at a young age.
He said he lived in 'squats, drug dens, [and] derelict buildings' for a year before he was privately fostered.
Moving on to his relationship with the victim, Hanley said he could only recall one argument prior to the incident, and said that ‘wasn’t really an argument’.
He told the court that Miss Harries called him into the bathroom that evening.
When he opened the door she was standing naked holding a towel, which she dropped to reveal a knife, Hanley told the jury.
“She thrusts the knife towards me while saying something [about] a woman named Jodie.
“I just full on grabbed it, just natural instinct.”
He told the court he then got behind Miss Harries to stop her from stabbing him, hoping to take the knife off her.
“We wrestle for the knife. The knife was swinging about.
“Sarah sustained some injuries when the knife has been swinging around the bathroom.”
The court heard that the ‘tussle’ spilled over into the kitchen and the knife ended up on the floor.
“We both scrambled for the knife.”
Eventually the struggle stopped. Hanley told the jury that at this point Miss Harries was walking and talking and was not crying.
He was asked how the knife ended up on the bed in the bedroom, but said the last time he saw it was on the kitchen floor.
Hanley said he left the flat in a panic, still unaware why the fight had taken place.
“My adrenaline was going. I was bleeding, there was blood everywhere.”
Hanley is due to continue his evidence tomorrow.
The trial continues.