"I thought I did this, but it turns out I only thought about doing it and then forgot to do it?"

The only word that comes to my mind is absentmindedness. As in:

"My friend did not receive my text because of my absentmindedness."

The action of being absentminded leads to this problem.

A friend of mine provided an interesting new phrase for this after some discussion. He proposed déjà fait, meaning (in his words) "something you thought you had already done." I think I like that best, short and easy to use. Have to see if it catches on now!

It's called C.R.S. "Can't Remember Shit". Many things are going through your mind at the moment, but what eventually happens is, you forget the most important thing that was on your mind.