How about reminding yourself that you're human. None of us are perfect, most of us get blindsided, for whatever reason, and end up back at "OMG, have I learned NOTHING?!?!?" (okay, that was ME a few weeks ago, but you get the drift).
Have a great time with ((DS)). If your ex doesn't remember he can have a longer visit, or that daylight savings time goes into effect tonight (at least for where I live)..his problem.
Though no one wants their child to learn a parent is unreliable, your ((DS)) will figure this out AND know that his mom is the reliable one. You're a great mom..enjoy your time with ((DS)) today and try not to even think about what may happen tomorrow.
Hugs and prayers,
Amy