Johnny Depp is "generous" and "loving" when sober but the "other side of him was a monster", actress Amber Heard has told the High Court in London.
Ms Heard said she had been "so in love" with her former husband and had "always held out hope" he would get clean.
But in a text to her mother, she said he was "violent and crazy" and she was "heartbroken that this is who I love".
Mr Depp, 57, is suing the publisher of the Sun over an online article that labelled him a "wife beater".
The newspaper insists it was accurate.
Meanwhile, Ms Heard's sister said in a written statement to the court that she "felt sick" when she learned the actress was going to marry Mr Depp.
Ms Heard was questioned by Sasha Wass, representing the Sun's publisher, News Group Newspapers (NGN).
Ms Wass read out text messages the Aquaman actress sent to her mother, Paige, on 22 and 23 March 2013, when the couple were in a relationship but not yet married.
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, who was married to the film star from 2015 to 2017, 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."
Ms Heard concluded her evidence by addressing Mr Depp's claim that her allegations were an elaborate hoax and that she had - in Ms Wass's words - "manipulated pictures, painted on bruises, destroyed property with your friends in order to implicate Mr Depp".
Ms Wass asked: "Has there been any benefit to your career since you have made these allegations against Mr Depp?"
Ms Heard replied: "No. What woman has ever benefited from being the victim of domestic violence?"
Ms Wass asked if Ms Heard had "benefited financially" from making the allegations, to which the actress said: "I married and divorced Johnny without a pre-nup in a no-fault state.
"I was entitled to 50% of his [assets] and he of mine without having to prove anything happened in the marriage, bad or good. It's a no-fault state and I was entitled to 50%."
Meanwhile, the court heard a written witness statement from Ms Heard's sister who said she "begged" Ms Heard, 34, not to go through with the marriage, telling her "him [Mr Depp] putting a ring on her finger was not going to stop him hitting her".
Whitney Henriquez said when she first met the Pirates Of The Caribbean star he seemed "very sweet" and she was "surprised how much I liked him".
The pair became close, "like brother and sister", she claimed, with Mr Depp, 57, calling her "sis".
But Ms Henriquez also said her sister's relationship with the actor was "tumultuous from the start".
Her statement said: "I wasn't aware of violence at the beginning, but I do remember Amber talking about them having big fights and being concerned for my sister.
"Amber told me that they would [verbally] fight - sometimes for days on end - and then go days without speaking to each other."
The libel case centres on an article published on the Sun's website in April 2018 under the headline "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.