Johnny Depp is “generous” and “loving” when sober but the “other side of him was a monster”, Amber Heard has told the High Court in London.
The actress said she had been “so in love” with Mr Depp and had “always held out hope” he would get clean.
But in a text, Ms Heard said being with the actor was like being on “a very fast train that’s about to explode”.
Mr Depp, 57, is suing the publisher of the Sun over an article that labelled him a “wife beater”.
The newspaper insists it was accurate.
Ms Heard, who is due to finish giving evidence on Thursday, was questioned by Sasha Wass, representing the Sun’s publisher, News Group Newspapers (NGN).
Ms Wass read out text messages the Aquaman actress sent her mother, Paige, on 22 and 23 March 2013.
In her first witness statement, Ms Heard claimed that in March 2013, Mr Depp hit her with the back of his hand, which caused her blood to hit the wall, after an argument about a painting by her ex-partner Tasya van Ree. He denies this.
The actress initially said the incident took place on 8 March 2013, but told the court she now considers the correct date of the “painting incident” to be 22 March 2013.
Ms Wass told the court that in one text to her mother, Ms Heard wrote: “It’s terrible, mum. I don’t know what to do.”
Another read: “He’s nuts, mum. Violent and crazy. I’m heartbroken that this is who I love.”
In a third, she wrote: “I feel like I’m on a very fast train that’s about to explode, but I don’t want to jump off and leave my love behind.
“So I stay on the train, even though I know it’s about to explode.”
Ms Heard then told the court: “I was so in love with Johnny at that time. We had had a wonderful year together where he was sober and clean and that is how I got to know him.
“And Johnny, when he was like that, (is) generous, loving, he is a remarkable man when he is like that.
“I loved him and I didn’t want to lose that… the other side of him was a monster, but I always held out hope that he would get clean and sober.”
As she read out the messages, Ms Wass told the court Ms Heard’s mother had suggested she informed her father, to which the actress replied: “I can’t tell dad.”
Ms Heard explained to the court: “I felt safe to tell my mother, but I didn’t know how my dad would react. I had never been in this situation before and I didn’t know if he would react violently… to Johnny.
“I didn’t want to make it worse.”
‘Trying to wake him’
The court also heard about a series of texts between Ms Heard and Mr Depp’s then assistant, Nathan Holmes, which were sent on 22 March 2013.
The actor is said to have been due on set at the time to film part of a documentary he was making about Rolling Stones guitarist Keith Richards.
Mr Holmes texted Ms Heard at around noon to say “on my way to get him”, to which Ms Heard replied: “Trying to wake him now.”
The court heard Ms Heard texted Mr Holmes just before 18:00 to say: “Success – he’s coming down.”
Ms Wass asked: “What was going on between noon… and about 6 o’clock that evening?”
Ms Heard replied: “Johnny refused to leave… (he was) snorting lines of cocaine, drinking whisky and he was saying we really need to work this out, he wanted to get to the bottom of it.
“He wanted me to admit that I was having an affair with, not only Tasya van Ree, my ex-partner, but also a gentleman I hardly knew… he had just gotten it in his mind that I had these affairs and he wouldn’t leave until I admitted it.”
Heard to conclude evidence
Ms Heard will conclude her evidence about 14 allegations of domestic violence, which NGN relies on in its defence of an April 2018 article that called Mr Depp a “wife beater”, at the Royal Courts of Justice in London on Thursday.
Her sister, Whitney Henriquez, who previously lived in one of five penthouses owned by Mr Depp in the Eastern Columbia Building in Los Angeles, is due to give evidence on Thursday afternoon.
The libel case centres on an article published on the Sun’s website in April 2018 headlined: “Gone Potty: How can JK Rowling be ‘genuinely happy’ casting wife beater Johnny Depp in the new Fantastic Beasts film?”.
The article related to allegations made by Ms Heard, which Mr Depp denies.