The 'Love Don't Cost a Thing' singer announced the shock split from her musician husband of seven years last Friday (15.07.11), but friends admit they had fought with each other for some time.
A source told usmagazine.com: "They had a very up and down marriage, they would get into heated, horrible fights.
"Then, they would also go through times where they were extremely loving and romantic. It was a roller coaster with them from the beginning."
Another source said the last few months had put real strain on the couple, who have three-year-old twins Max and Emme together.
A friend told People.com: "Marc and Jennifer decided it's best to go their separate ways for the sake of their two kids, after months of nonstop arguing."
The couple had reportedly "tried everything she could to make it work," but not even therapy sessions helped.
Another source said: "They went to marriage counselling. She was patient and loving and did everything he wanted her to. She exhausted all the possibilities."
"She is devastated. Absolutely beside herself. This is someone she dedicated a long time to, and that she truly loved."
Jennifer, 41, is thought most likely to keep primary custody of the twins and is said to be trying to remain positive and keep herself busy with work - which includes her re-launched pop career and acting in new film 'What To Expect When You're Expecting'.
Another friend said: "She is looking forward to the future. She is jumping back into work and being a mom with all her energy and she is really excited.
"She feels like she has grown leaps and bounds and is a new person now, better than ever."