"Looking back at it all, it's tough to watch," the reality star, who got engaged to Kaity Biggar in Monday's Bachelor finale, tells PEOPLE in this week's issue
He thought the rule would keep things from getting messy with finalists Ariel Frenkel, Gabi Elnicki and now-fiancée Kaity Biggar. And they did after Shallcross, 26, decided to be intimate with Elnicki, 25, on their overnight date.
"Looking back at it all, it's tough to watch," Shallcross tells PEOPLE in this week's issue. "It was the farthest thing from perfect. I was trying to make it as smooth as possible, and in the end it really just bit me in the ass. It bit everyone in the ass. And that's something that I will always remember, but work to get passed. Those actions don't define who I am."
On the show, Shallcross told Biggar, 28, about how he broke his own rule to be with Elnicki, and it put a damper on their Fantasy Suite experience. Eventually, they moved past it.
"That episode didn't go the way that I thought it was going to," Biggar tells PEOPLE. "But I think at the end of it, we came out stronger and that's the only thing I could have wanted at the end of that situation. So it was a hurdle, we got through it and I'm proud of us for doing that."
Shallcross calls the conversation he had with Biggar that day "really important" to their relationship.
"The biggest and most important thing was making sure that she was okay and she was understanding where I was coming from," he says, "but also just growing and mending what had happened and becoming stronger for it."
Frenkel, 28, told host Jesse Palmer that she learned about Shallcross's decision to go back on his word only when watching last week's episode. She wanted to know why "I wasn't given that same respect" as the other women who Zach did tell about breaking his own rule.
"You're right," Shallcross admitted. "I should have told you, probably first. I just felt in that moment that I've got to focus on everything I can right now in the moment and with how that week was set up with having the next date with Gabi, there was just a lot of stuff going on in my mind. And I do owe you an apology for not telling you at all and you found out when the rest of the world did and that's on me."
Later, Elnicki, 25, told Shallcross she felt "blindsided" by his choice to share the details of their overnight date with Biggar.
"We decided that it was going to be between us," Elnicki said to Shallcross. "We said that that night. And I remember I was brushing my teeth, you came up behind me, you kissed me on the back of my head, you said, 'This is just between us.' And I said, 'Yes, this is just between us.'"
She called the fact that the world knows about her intimate night with Shallcross "extremely violating."
"My mind felt like — which is wrong — I felt like I needed to let everyone know that I went against my word," Shallcross explained.
He also confessed, "There's no excuse for the way I handled things."
Following his three Fantasy Suite dates, Shallcross kept Biggar and Elnicki around and sent home Frenkel. He ultimately proposed to Biggar in Monday's Bachelor finale.
The senior account executive suspected from the start the Biggar could be the one.
"She, from the very beginning, was someone that just exuded everything I was looking for — that kindness, that loving, funny, sweet, independent, beautiful person," Shallcross says. "I always felt so pulled and attracted to her, and she was someone that throughout the entire process, it was becoming so easy in my mind that this person is my future and I want to do life with her. I just absolutely fell in love with her and I was like, 'I can't lose her. She's mine.'"
Shallcross recalls being anxious before asking Biggar to marry him.
"I remember standing on the podium and seeing Kaity walked towards me and my knees were shaking, my heart was in my throat," he says. "I was the happiest I could have been just seeing her there and getting down on the one knee. It was a really unreal moment and I often replay it in my mind."
Biggar had been going through a similar experience on proposal day. "It was an extremely nerve-racking day," she says. "I remember walking around the corner and seeing Zach smile. I was like, 'OK, hopefully it's me. This seems promising.' But I was shaking in my boots for sure. I was sweaty, but it was such a special moment. I was elated, truly."
The couple feels like going through what they did on The Bachelor has only strengthened their relationship.
"It's a weird experience and it's not easy, but it's something that we do together pretty well," Shallcross says. "It's been where we have grown stronger and overcome some tough things together, and we've come out better and stronger for it at the end. And that's something that I'm so appreciative of. It makes our relationship so loving and strong."
For more on The Bachelor finale, pick up the latest issue of PEOPLE, on newsstands Friday, or subscribe here.
Schallcross calls Biggar "a great communicator" and says their relationship has trust and honesty. Now, he can't wait for "the start of the new chapter in life" with Biggar as they look forward to living together in Austin.
"Right now, we're just enjoying each other's company and getting to know each other more," Biggar says. "Zach and I want this to work out."
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