A "bitter" Amanda Marsh says she ended her relationship with The Bachelor star Alex Michel when she discovered he had slept with runner-up Trista Rehn during the reality show last spring.

"I couldn't get over some things that happened on the program," Marsh told last night's Inside Edition. "Some of the girls from the show had called me and told me that Alex had slept with Trista, which I did not know."

According to Marsh, Alex and Trista hooked up while on a trip to Hawaii that was featured in the show.

Marsh said that, when she confronted Alex, "He's like, 'Yeah, I was intimate with her.' And so I had to get over that and then . . . everything just kept eating at me.

"And it just wasn't all making sense."

Trista, who was snubbed by Alex in The Bachelor finale, now stars in The Bachelorette, a turn-the-tables take on the show in which Trista will choose a (potential) mate.

Marsh told Inside Edition that, even though Alex chose her, he hadn't completely gotten over Trista.

She knows this because, just days after she broke up with Alex, she received an e-mail from Trista, Marsh said.

In the e-mail, Trista revealed that just weeks after The Bachelor finale, Alex had told her to "keep the door open" on the possibility of the two reuniting down the road.

"I'm just very bitter, I guess," Marsh said. "I'm disappointed because I invested a lot of time in trying to make this relationship work."

It was widely believed that Marsh and Alex hadn't been a couple for quite some time, despite their public protestations.

So what's Alex's reaction to all of this?

Apparently, not much.

"We shared so much of our personal, romantic lives with America and everyone was so supportive and seemed to think I'd picked the right one and everything," he told Inside Edition.

"But there came a time we had to be private."