Pre, Pro, what the heck!

I was corrected by Word when I wrote "proceed" when I meant to say precede. I was impressed by Word's ability to recognize that I had used the incorrect word and thankful that it save me from embarrassment (although I am not admitting to the embarrassing behavior). I said to myself, "You dummy pre- means before and pro means moving forward or after." Then... I wrote the word proactive... Proactive means to do something BEFORE to prevent something from happening in the future. Now why is this word not Preactive?

Additionally, why is -"ceed" (or "cede") different in proceed and precede?

Good grief!