Save me, save my kids

Can anyone help me NOT freak out when DD has an accident? OMG, I was doing so well not losing it with the kids. I have actually found some patience with them lately. Until last night, I was upstairs doing the usual girl thing before we head out to dinner. I was thinking, "I should go downstairs and make DD go potty. It's been awhile. Ok, right after I put on my lipstick I'll go take care of her." I guess my own vanity could be what got me upset but next thing I hear is a sad "Mooooommmmmmyyyyy!!!!! Ohhhhhh I had an accident"
A few select words came flying out of my mouth before I raced down the stairs and said, "Where?!" "On the couch again." A few more choice words. In the playroom we have a great futon cusion on the platform that has hinges and serves as the couch and bed. Thankfully the fairly waterproof cover comes off. Thankfully I have a good carpet cleaner with upholstery attachments. Regrettably, it's a big PITA to deal with it all.
And DD got in major trouble. She had an accident because she didn't want to stop watching the SpongeBob cartoon she was engrossed in. So she lost cartoons today for it.

When I am calm and not cleaning up pee, I KNOW that she doesn't do this on purpose. I know that she's not sitting there thinking, "hmmm, Mom's had it a bit easy this week, I think I'll pee on the couch so she's got something to do. I know how much she loves to clean this couch." I know she doesn't want to get in trouble and feel all icky.
But when she does it I just completely LOSE IT. I mean, screaming and cursing and furious. I scare her, I scare DS, heck I scare me! I have got to find a way to calm down when this happens. Rationally, I know it's not good for her, for me or anyone. I know all of the psychobabble stuff about it. In the heat of the moment though, my brain goes down the drain.
I do think I will pick up a waterproof mattress cover for it though. The waterproofing is sure to wash off the decorative cover if I keep tossing it in the washer once a month!