PR executive Kate Greville said in the lead-up to the alleged assault in November 2020 she had learned he had "full-on relationships" with eight women while they were together.
The discovery of messages "going back years" on the former Manchester United footballer's iPad prompted her to decide to leave him, she said.
Giving evidence at Manchester Crown Court from behind a curtain, Ms Greville said she had secretly signed up for a flat and planned to "disappear" from Giggs' home in Worsley, Greater Manchester, where she had lived since the Covid-19 lockdown in March 2020.
On the afternoon of November 1 she went with 48-year-old Giggs to Manchester's Stock Exchange Hotel, part-owned by the defendant and fellow ex-footballer Gary Neville.
But an argument followed at dinner in the restaurant, she said, when she told Giggs about a man who had previously asked her out for a drink.
She told the court: "I said to this guy 'I can't believe you have asked me out, you have a girlfriend'."
Asked how Giggs responded, she said: "It was 'why didn't you say you had a boyfriend?' He was angry. He kept having a go at me. He wanted me to agree with him. I was not prepared to do that. I stood up for myself."
She said he "stormed out" and up to their hotel room where they had planned to stay overnight.
Ms Greville, who is 36, said she stayed at the table and felt "a bit embarrassed" before she went up to the hotel room.
She said: "He had packed up all of my things and said he wanted me to get out.
"He was basically saying he was finishing with me and he didn't want anything more to do with me.
"I knew everything that he had done.
"I had the proof for the first time ever because he had always managed to convince me the whole thing was in my head and I was going crazy.
"I showed him some of the messages I had seen.
"I had taken screenshots, there was a lot.
"He said what I was saying was not true.
"I said 'it's there, you can't deny it any more'.
"Then he got in my face and screamed and told me to get out.
"He was very drunk and very angry."
She said she collected her belongings and called for an Uber to Giggs' address where her sister, Emma, was looking after their golden retriever puppy Mac.
Ms Greville said she phoned her and asked her to "get my stuff and the dog" into her car.
She returned and was upstairs packing before 10 to 15 minutes later she saw lights outside as Giggs arrived and started unloading her belongings in bin bags from her car at the front of the house, she said.
Ms Greville said: "He was very angry. I had not seen him that angry before. He was drunk. I think the whole situation had sobered me up.
"I was saying, 'Please. I just want to go to my dad's. I just want to take the dog'. I was obviously very upset, he was my absolute world that dog."
The pair then grappled over Ms Greville's phone in the hallway of the house and inside an adjacent utility room, the court heard.
Ms Greville continued: "He pushed me backwards, I ended up being on my back in the utility room.
"I was on my back on the floor with my phone in my hand.
"Ryan was on top of me.
"He's pinning me down on the floor and trying to reach for the phone in my hand.
"I was trying to do everything I could to get him off me. I was shouting my sister for help, 'Get him off me.'"
Ms Greville said Emma grabbed Giggs by the waist to pull him off but he pushed his arm back to try to get her off him and it is claimed he had hit her in the jaw.
Giggs managed to grab Ms Greville's phone, pushing her head into the floor as he got off her, she said.
Ms Greville said Giggs told her he had called the police and was going to tell the officers Ms Greville had attacked him and was a "psycho".
She added: "He was really angry but quite with a cockiness to it.
"There was a certain baiting, he was trying to bait me, basically."
They then moved to the kitchen.
Ms Greville said: "I said 'I'm glad the police are coming because you attacked me and I will tell them what happened'.
"That's when he came towards me and headbutted me in my face.
"He just came at me, grabbed me by my shoulders and headbutted me in my face.
"I said, 'I can't believe you have just done that. I was in shock. I fell backwards, my lip instantly swelled so I put my hands to my mouth. I could taste the blood.
"I was just in shock.
"All the other times he had hurt me, this was different. This was with real intent. He really wanted to hurt me."
Ms Greville said Giggs then said to her: "You need to think about this, Kate.
"This will ruin me and it will ruin you.
"Think about my job, think about my career, think about my kids.
"And I said, 'Well, you shouldn't have done it, then'."
Police attended and Giggs was arrested.
Giggs denies using controlling and coercive behaviour against Ms Greville between August 2017 and November 2020, assaulting her, causing actual bodily harm, and the common assault of her younger sister.
Ms Greville will continue to give evidence on Wednesday.