"We started talking in January 2016, just through direct message. We kind of talked off and on just through Instagram, but I landed a job in Scottsdale after graduating [Miami University in Ohio]," she explains. "I flew out to search for apartments, so I messaged Arie and I told him that I would be in the area and he asked me out on our first date."
"We hit it off right away and started dating as soon as I moved here," she adds. "To my knowledge, the relationship was monogamous."
The blond beauty tells ET she and Luyendyk Jr. went on their first date in July 2016, and recently celebrated their one-year anniversary by going back to the restaurant where they first met: Chelsea's Kitchen.
She felt their relationship was serious and would like to think he did too. When ET asked Stempfley if any cheating was involved, she said, "I mean, I would hope not."
"I’m not assuming anything, I just... that’s all I know, is that at least on my end, yes, the relationship was monogamous," she explains. "We always had a great time, tons of fun. We made each other laugh and had just really a great relationship so I can’t complain there… [we] had great memories."
Stempfley explains that she met Luyendyk Jr.'s dad the second day that they hung out and he had met her family when they were in town visiting. The former couple also introduced each other to their friends as boyfriend and girlfriend. She says they even spent holidays together.
“I attended all of his family holidays including Thanksgiving and Christmas," she claims, adding that just within this year, she traveled with the reality star to three of his out-of-state races.
“My family was head over heels about him," she shares. "They thought he was very charming and very sweet and they really liked him. [Arie and I] traveled together and I went to his races to support him. And he also actually lived with me for a few months while he rented out his house back at the beginning of the year."
"He typically rented it out during race season, so he was kind of gone a lot," she adds. "But when he was in town he stayed with me and he asked [if he could]."
Flash forward to July 31, 2017, when everything seemingly changed overnight. A source previously told ET that the couple broke up just two days before Luyendyk Jr.'s Bachelor announcement, but Stempfley wanted to make clear that's not the case -- they had actually broken up weeks before.
"The night before the breakup, we hung a light fixture together, made beef stew and I slept over. Everything was normal! The next day, we had plans after I got off work. It was just a regular day, so it definitely caught me by surprise. It was over the phone originally, just out of the heat of an argument, just a small argument," she claims. "But then I went over to his place and we discussed it further and that’s kind of when it happened [for good] -- it was in person."
"It was very emotional," she adds. "You know, you’re trying to get answers and you can’t. I couldn't really get a definite answer … I've never seen him cry, ever. I could tell that he was emotional, but there wasn’t tears on his end."
Stempfley claims the pair’s last romantic night out together was their one-year anniversary dinner on July 19, 2017, less than a month before she picked up her belongings, like kitchen appliances, hair brushes, and bathroom essentials, from his house.
And when it came to learning that Luyendyk Jr. would become the season 22 Bachelor, Stempfley says she found out just like the rest of Bachelor Nation: during his appearance on Good Morning America earlier this month.
"Backing up, I saw Arie's name being tossed around on Twitter, suggesting he should become the next Bachelor. So I texted him for the first time in awhile, exactly four days before the announcement on Labor Day, and I said, ‘Are you awake?' And he called me within a minute or two," she claims. "I asked him, 'If you were becoming the next Bachelor, would you at least have the decency to tell me?' and he said, 'Yes, of course,' and then proceeded to tell me that he hadn't been contacted by producers, so he doubts that would even be a possibility."
"So, when I found out he was becoming the next Bachelor a month after our break up, I'm sure you can imagine that I was completely blindsided," she adds.
When ET asked if she thought The Bachelor had anything to do with their breakup, she says, "I would like to think it didn't."
"You can only believe what someone tells you, so, you know, there’s no proof that he had been auditioning or talking to producers at that point," she explains. “The only thing I have to go off of is his word."
As for what advice she'd like to give the girls who'll be competing for Luyendyk Jr.'s heart?
"Good luck," she jokes. "You know, I really don’t have anything else to say to them... I don’t know what’s going to come about this season. If this is what he wants and if he's determined for it to work, then I hope that it does for him."
"[But] I don't want to sound like I’m bashing him," she continues. "The breakup has been absolute hell for sure, but aside from this, he is a really great guy. We had the best relationship and that's why it's so shocking to me when this [came] out and then you hate to question [the reason] because of his word but at the same time..."
Following their breakup, Stempfley tells ET she had actually applied to be on the show after her friends encouraged her to audition. She says she didn't get that far in the process, and "had no idea" that Luyendyk Jr. was going to be the next Bachelor. She says her ex was not aware she applied.
"I applied just about a week or so ago actually, not too long ago," she clarifies. "I leaned on my girlfriends for support through this time and at one point they suggested that I apply for the show."
"It had never really crossed my mind but I decided to do [it]," she continues. "So I actually only submitted an online application. I didn’t get around to making a video, so I wasn’t far into the process."
Stempfley says ABC has not reached out to her about possibly coming onto the show as a suitor or guest. But if they do, would she do it?
“I would need to think it through before coming up with a definite answer," she says. "But I do feel like I have questions, of course, [like], 'Is this the reason? [Is this] why you broke up with me?' That's probably my biggest question."
As for what went wrong, Stempfley says she's "not totally sure" and hopes that she'll one day get closure.
"I loved him and he led me to believe that he loved me too," she admits. "We never said it, I think probably because we’re both very stubborn, but I thought the feeling was mutual. All in all, I guess we were just progressing at different rates."
“We really stayed focused on living in the moment and enjoying the time we spent together, so [it was] just one of those things we didn’t really talk much about," she says, when asked if the two ever talked about having kids or getting married in the future. "[He told me] that after a year of dating, his feelings should be stronger than they currently are."
ET has reached out to ABC and Luyendyk Jr. for comment. ABC is not commenting.
For more on his casting announcement, watch the video below!