Eighteen months ago, Sarah went snooping on her husband's computer and opened a deleted file that made her feel as if she'd been whacked in the stomach with a baseball bat.
It contained an exchange of love notes with another woman.
"It is the most devastating thing you could ever imagine," she says. "You can't believe what you're reading, it can't be true, but of course, part of you thinks, 'Why did you look? This is finally making sense.'"
It explained his fidgety, nervous behaviour; why he'd suddenly change screens or shut down his computer when she walked into the room and asked him what he was doing; why he often stayed up late on the computer after she'd gone to bed; and why she sometimes couldn't reach him during the day and had no idea where he was.
Sarah angrily confronted him and told him he had a problem. He seemed relieved to have been caught, she says. He told her the woman meant nothing to him, immediately cut off further communication with her and disconnected his Internet account.
Two and a half months later, he seemed twitchy again and Sarah confronted him a second time, found he was back at it, and told him to get out.
"I felt so betrayed. I was hurt. I wondered, what's wrong with me that he would do this?" she says. It was faulty thinking, and she would later realize it had nothing to do with her and she was not to blame.
Her husband is her best friend, but when she was most heartbroken, she couldn't go to him and cry in his arms because he's the guy who hurt her.
Sarah couldn't eat, couldn't sleep, lost weight, cried a lot, and still had to care for four little kids and work a full-time job.
A friend asked if Sarah's husband might be a sex addict and directed her to a website with a questionnaire, which Sarah forwarded to her husband, who was staying at a hotel. She told him to check out the site and get back to her.
He called back five minutes later and said his responses to the questionnaire were off the charts. Two days later, he was in therapy for sex addiction.
Sarah knows what some of you are thinking: "There's no such thing as sex addiction; it's just an excuse for cheating," and "You must be a doormat, staying with a man who's been unfaithful. How can you ever trust him again?"
You've obviously never had an addiction and don't know anyone who has, she says.
"It's not just somebody going out and having dozens of affairs and saying, 'I'm a sex addict'; it's a real illness."
Her husband is a textbook case, she says.
He came from a home where there was no bond between him and his mother. His parents were very domineering, not nurturing, not loving, and they shamed him. Everything he did was never good enough.
Like many sex addicts, he learned to soothe himself in a sexual way, and when he became an adult, he was always looking for a short-term sexual relationship with someone to give him a high, Sarah says.
It has nothing to do with looks, which explains why sex addicts bed other people, even when they're married to beautiful people such as actors Halle Berry or Tea Leoni.
People think sex addicts are oversexed, but they really feel ashamed and can't understand why they do what they do, Sarah says. That's why most don't find out they're addicts until they're caught by a spouse or partner and are forced to confront who they really are.
Some people also think women who decide to stay with their partners after infidelity are weak and have no self-respect, but it's actually the reverse, she says.
"If Elin (Nordegren, wife of golfer Tiger Woods, who has been reported to have spent time at a sex-addiction treatment centre in Hattiesburg, Miss.) decides to stay, I think that makes her powerful, makes her a very strong person, because she still wants to salvage something she believes in," Sarah says.
"If I didn't like my husband and he wasn't my best friend, it would have been easier to walk away than stay. He's also a good dad, and the kids don't deserve having their dad taken away from them because of an addiction."
Her husband completed a 12-step rehabilitation program and carries around a chip, much like those carried by reformed alcoholics, that says how long he's been clean.
There's no cure for addictions, Sarah explains. "For the rest of your life, you're an addict in sobriety."
Sarah, too, is in therapy. She's learning that a sex addict's partner usually comes from a family where one of the parents was an enabler. "So we're fixer-uppers, usually in professions such as nursing, teaching or counselling. We think we can save the world and our love will make everything better."
She's also learning how to trust her husband again. He is showing her he can be trusted by blocking sites that stimulate his addiction, and by being totally transparent and allowing her to randomly check his computer, his e-mails and his phone. He isn't allowed to delete anything unless she knows about it. And she must be able to reach him whenever she tries, and know where he is at all times.
"He always says, 'I'm so sorry my past has caused this so that you feel this way. You're the last person I would ever want to hurt and I'm so sorry I've brought this turmoil into your life.'"
He's had about three slips since acknowledging his addiction, and he's admitted them, as required, to Sarah, his therapist or therapy group.
"He's an amazing husband, father and partner in so many ways," Sarah says. "Our marriage has got so much stronger, but I don't trust the addict in him with my life or my family's life."
If she ever finds out he's been lying to her and has been relapsing, "at that point, I would walk away, because it's not healthy for me or the kids or the family," she says.
"It's been hell sometimes, but I've learned a lot and I can depend on myself if anything goes wrong. It would be hard to lose my best friend, but I would have to look after me and my family and move forward."