Dic: get ahead of oneself

• I have to stick to my notes or I will get ahead of myself in my lecture.
• When he bought a new little bicycle before the baby was born, he was getting ahead of himself.

Fig. [for someone] to do or say something sooner than it ought to be done so that the proper explanation or preparations have not been made.

.... McGraw-Hill Dictionary of American Idioms and Phrasal Verbs