Why do I always do this?

I concocted this great scheme where I would meet Hubby's mother halfway (they live out of state) and our son could stay the weekend with his grandparents and we would have this wonderous adult weekend that we have not had in over a year now. Friday night would be going out to dinner and dressing up, Saturday going to see P!rates II and going out with friends somwhere and Sunday brunch with my mom and her guy and then sushi with my girlies at 5. Sounds fun filled and WONDEROUS right? Wrong.
I drove my little buddy, met MIL just fine, and then sobbed heartbrokenly for about 30 minutes driving home, and then off and on the rest of the day. I had a horrible headache when I finally got back home and so I laid down, ended up sleeping until Hubby got home which was not my plan, and then was completely groggy, grumpy, and still sad. Bitched and griped way too much over the course of the evening, ate far too much-making myself sick, and then the two drinks I had kicked my ass all over the place. Now why on Earth did I think this would be fun? Overeating rich, salty food is not fun to me anymore (though I keep trying it again and then getting mad) and the whole thing just sucked. MEH.
I woke up off and on all night because I was so dehydrated from not drinking enough water yesterday and then eating the overly salty rich food, and I've downed at least 3 glasses of water now and am still sitting here with cotton mouth and grumpiness. Did I mention the 3.5 lbs of water weight (I hope)?
Today just has to be better, why can't I enjoy being a young person?