Sources close to the volatile couple tell TMZ ... both Ronnie and Jen feel like the domestic incident from last week was the straw that broke the camel's back -- albeit for different reasons. Still, we're told they realize they can't continue on in a relationship.

It sounds like they've both reached this conclusion separately, as our sources tell us, each person has been telling their own group of friends the same thing in terms of breaking up.

While there may be disagreement on who's ultimately at fault, it sounds like Ronnie and Jen are on the same page as to why they gotta end things ... their daughter's safety is jeopardized when they're together, and they can't risk that anymore.

From Ronnie's side, we're told there's no going back from this fight despite having worked it out the past ... as he feels things just went too far. Ronnie's been saying it's over.

Ditto for Jen -- who we're told is also concerned about their daughter's well-being and is vowing to never put her in a situation like that again. Adding insult to injury ... our sources say Jen still plans to seek a restraining order against Ronnie for her and Ariana.

Remember, Jen was viewed as the victim this time around -- as opposed to the culprit, like she had been in the past -- which led to Ronnie getting arrested for kidnapping. She claims he battered her during the ordeal, and her alleged injuries were gnarly.

As far as custody, we're told Jen and Ronnie haven't discussed anything new regarding their current arrangement -- 50/50, more or less -- but our sources say a new deal with new terms will probably have to be worked out down the road.

Of course, Ronnie and Jen have been down this road before ... but have always gotten back together one way or another. Even with several incidents -- including a ransacking romp, an ashtray-throwing moment and even a car-dragging episode -- they always got back together.