Going back to the origins of the word itself might help (or it might not, but it makes me feel better). In the ancient Greek theatre:
proto = first
agonistes = actor
So, the protagonistes was the actor who took the biggest part, the one who was on stage the most. Other actors might play multiple roles (in classical tragedy there were only 3 actors in total, plus the chorus) but the protagonistes was on stage for most of the play and generally only played that one part. The main character was often the one the play was named after - Agamemnon, Antigone, Philoktetes, Elektra, Medea etc. The story revolved around them, so that even when they were not on stage they were never far from the audience's mind. The other characters talked about them, anticipated their return, pondered their actions. The protagonistes moves all the action and dominates the theme of the play.
Now, something else about the origin of the 'agonistes' part - it comes from the word 'agon', which means struggle, contest or combat*, so an agonistes also means a combatant, someone who struggles for something, a champion.
When you think about what really drives a story, it is tension or conflict. Your antagonist is the one in the thick of that conflict, driving it with their needs and desires.
Hope that helps.
* To give the other side of the coin, 'agon' also means speech or argument, which has obvious links to the theatrical context as well.