Kevin Federline continues to make bombshell accusations about Britney Spears! His highly anticipated memoir, You Thought You Knew, hits bookstores next week on October 21. But the tea is already spilling out! In a new excerpt published by USA Today on Wednesday, the former backup dancer opens up about the rocky start to his relationship with the pop icon. They met and began a "whirlwind" romance in April 2004 - right before the European leg of The Onyx Hotel Tour. According to Kevin, Britney told her team she refused to head out on the road without him, so he joined her. Related: Kevin Caught Britney On The Phone With Justin Timberlake Before Their Wedding! However, life on tour was difficult for the couple because he caught her cheating on him pretty soon. It was in Amsterdam... and it was with a woman! OMG! Kevin writes: "It was supposed to be unforgettable - iconic city, vibrant nightlife, and the kind of freedom you can't find anywhere else. But for me, it became unforgettable for other reasons." It all started when he, Britney, and some of her tour dancers and staff all went out in the city - including Teresa Espinosa, who first introduced the pair at a Hollywood club called Joseph's. But her role in the story is more than matchmaker... He recalls: "It was one of those loose, carefree evenings where everyone was dancing, drinking, and just having a good time. We stayed out late, hitting one of the city's nightclubs until at least one or two in the morning. It felt like just another fun time on tour - a night you laugh about later." Britney and backup dancer Teresa Espinosa in concert. / (c) WENN But K-Fed isn't laughing about this night! The group eventually returned to the hotel. Britney and Kevin went back to their room, but she left shortly afterward. The actor "didn't think much of it" at first, figuring "she was catching up with someone or handling something tour-related." But eventually he began to "wonder what was taking so long." He continues: "Eventually, I decided to check if everything was okay. I stepped into the hallway, our door still cracked, like most on the floor, and headed toward the elevators. As I passed one of the rooms, I glanced inside - and froze." What did he find? Kevin claimed he caught Britney cheating on him! With their friend Theresa! Whoa! He recalled: "Theresa was sitting on the edge of the bed. Britney stood between her legs, hands on her face, and they were [expletive] going at it. Full-on making out. It was one of those moments that hits you so fast, your brain can't even begin to process it. You just stand there, stunned, not sure what's real. I didn't say a word. Just stood there, watching. For a second, nobody moved. Then they both looked up and saw me, and I turned and walked away." And in that moment, Kevin writes, he realized he needed to pack his bags and leave: "My heart was racing, but not with excitement. It was something colder, a sudden clarity. This wasn't what I signed up for. So I got the hell out of there. As soon as I got back to our room, I grabbed my suitcase and started packing. At that point, I cared about nothing else - I just wanted out. I picked up the phone, called Fee, and asked her to book me the first flight back to the States. I didn't care what time it was. I was ready to sit at the airport all night if necessary. I was done." When Kevin finished packing, the singer returned to the room and begged him to stay: "She knew immediately, just by looking at me, that something was wrong. 'What's going on?' she demanded, her voice laced with panic. 'I'm leaving,' I told her, my tone cold and final. 'You can do whatever you want, but this isn't what I came here for. If this is how it's gonna be, I'm out.' In an instant, her expression shifted from confusion to regret. She started apologizing, explaining, promising it would never happen again. She told me she didn't realize how serious I was about us, that she was overwhelmed, and it had been a mistake." Kevin "could tell she meant it," but he was "still furious" and "hurt." He made it clear he wasn't interested in this being a throuple situation: "I told her that I wasn't there for the 'extra [expletive],' as I called it. I wasn't interested in a relationship that involved more than two people. I wasn't judging her. I was simply laying down boundaries. I wasn't going to let this set the tone for whatever we were building." Wow... Ultimately, Kevin did stay and give her another chance. And things changed between them from then on: "I didn't think about it then, but that moment was a turning point for both of us. It was our first fight, or whatever you want to call it. I think it was the moment she realized I wasn't there for the fame or the perks. I was only there for her. The fact that I was ready to walk away, even after everything I had already given up to be with her, proved it was real. In the days that followed, something seemed to change between us. She became more intentional, more thoughtful ab