Four weeks after giving birth, Petria saw her husband's phone bill. Everything crumbled.

Petria was 18 when she met the man she would later pledge to be with forever. They were young – just teenagers – but both with an insatiable appetite for travel and adventure.

“I was quite young… to be honest, it was like any young romance – it’s not that we were young, dumb and stupid, but you do fall in love very quickly and move very fast,” the now 32-year-old tells Mamamia from her home in Brisbane.

Within a year of being together, they had moved overseas. Within two, they were married.

“We moved over to the UK within a year of being together. We spent two and a half years there and we just lived and worked and travelled.”

At the age of 20, she was married to a man she had known for two years. Her family, she says, were happy. They were happy if she was happy. But it was when they moved back home to Australia that everything changed.

“Within three months of being back in Australia, I was pregnant which was a bit of a surprise. A happy surprise, but a surprise. It was a total life changer.”

They were young, but they were excited. Baby Rhiana arrived when she was just 22. Four weeks later, she found out her ex-husband had been having an affair throughout the entirety of her pregnancy.

She found out in a way that, even today, isn't uncommon: The phone bills.

"You'll always remember [that moment], because it was so life changing and so heartbreaking. I was checking our phone bills and there was a lot of international texts to the lady. I immediately questioned it because I just couldn't understand, we didn't know anyone in South Africa. It turns out the lady was on holidays in South Africa.

"He denied it, like most instinctively do. [But] there was one person who was quite close to him, and I went straight to them. They confirmed it."

Petria's husband had been having sex with another woman while she was at home, carrying their baby in her belly.

More than one person in her ex-husband's immediate circle knew of the affair. No one went to Petria, at home with a four-week old baby, and told her about it.

"They thought it was a ticking time bomb, they knew it was going to come out at some point, they just didn't know when.

"As anyone who has been in a young relationship, I did think it was going to be forever. I still believe everything happens for a reason, that whatever happened at that time was meant to happen, and I don't regret anything I've done, but if my daughter told me she was getting married at 20... well, I wouldn't recommend it.

"I believed that I knew everything about him, and I did, to a point."

Despite her ex-husband's repeated denials, the couple immediately split. They sold the unit they lived and owned together and she went home to live with family.

"When we first separated, I felt it did taint my entire pregnancy, but that was just my initial grief. That was the first stage. Now, I don't feel like that about the pregnancy at all, but at the start it tainted it."

However, it influenced no part of her relationship with her daughter. If anything, their bond was "made much stronger", despite the fact that looking back, the affair - in her opinion - wasn't an isolated event.

Esther Perel tells Mia Freedman all about the psychology of cheating. Post continues after audio.

"I think in hindsight, you can pinpoint certain moments and other times it probably happened. I've never questioned it since, I just don't really want or need to know."

As for the affair that broke up their marriage, Petria says it was never formally confirmed on the behalf of her ex-husband. He, for the record, is still with the woman. They have their own children together.

"It was all basically confirmed when he... moved in with her soon after we split and when he was having visitation, he would bring her along.

"It's completely heartbreaking, I won't lie. It was definitely a horrible experience in my life, but as I said, everything happens for a reason. I have since re-married and I met [my husband] eight months after my split. I can see now that I was meant to go through through all of that to meet him."

After meeting her now-husband Ben through a mutual friend, Petria says it "took some time" and "a lot from [her] side" to trust someone again.

"As any person who goes through a breakup, it took a lot from my side to trust someone again. It takes a lot to build that trust back. But my husband now is totally the polar opposite to my ex-husband."

They've since had two children together - Lola who is three and Coco, aged one - and she and her ex-husband are, remarkably, on really good terms.

"Our relationship is so good and compared to what it used to be... we have taken some family holidays together. At the end of the day, it's important forgiveness happens on both sides because going through a divorce and splitting money and everything was quite toxic. But it's definitely good now. In fact, I definitely deal with [the other woman] now more than him. I have a good relationship [with her] now, though I would have never expected that.

"There was no actual ‘date’ as such when we realised that we were on better terms… both my ex husband and I had noticed increased co-operation, a more free-flowing exchange of communication and leniency once we both started having children of our own with our respective partners. I'm not sure if we realised there were more important things to consider than arguing over the little details or we both got too busy but our situation definitely calmed down from there on."

She said while their divorce was "very toxic" and she dealt with "some really painful emotions", the two have cultivated a relationship that's healthy and co operative and one her daughter can look up to.

"First and foremost the children need to be put ahead of either parent's own interest. If nothing else you both have an intense common love for your kids that you share, so keep focus on the greater good for them. If your ex partner gives a nice gesture, make sure it is acknowledged and return the favour.

"Never argue in front of your children and if need be, send carefully worded emails or messages in preference to discussions that can blow out into heated arguments. Start with small gestures and build trust. Have lots and lots of patience and most of all you'll both need to learn forgiveness. It's paramount in order to move forward. It probably took us six years to make any noticeable change."

No matter what, Petria stresses, bad things always have good outcomes. It might just take a while for  them to surface.

"No matter what anyone goes through, there is always an outcome. If you've got the children as the main focus, the adults can learn to get over themselves. Rhiana's siblings just all think they're one big family, and if people can learn to forgive, then everyone is better off."