Short version: it's all about the visual cues. Watch something on the wall in front of you until your knees block your view of it. Watch the knees go over top, then look for the landing.
Long version: your brain's natural intuition about how this skill should work is playing a trick on you. Everything your brain uses to track how you're oriented, how you're rotating, etc, is in your head. And I don't mean "in your head" as in it's not real; I mean "in your head" as in it's between your neck and your hair.
So when your head rotates, your brain thinks that makes the rest of your body rotate along with it; conversely, when you leave the floor and your head isn't immediately rotating, it's intuitively scary because your brain feels like the rest of your body isn't rotating either.
So when you take off with the intention of flipping backward, your intuition is to throw your head back, because that's what makes you feel like you're flipping.
The way to short-circuit this is to give your brain something to focus on that will override that intuition. And the easiest and most intuitive way to do that is to focus on visual cues. By watching the wall in front of you until it is physically impossible to continue watching it, you force yourself to keep a neutral head position on takeoff, and give yourself an immediate way to know whether you did it or not.
Did you not see the wall at all? Then you probably dumped your head back. Did you glance the wall briefly? Then you probably had decent position on takeoff, but let the head go early when you left the ground. Did you get a good clear look at what was in front of you, and then see your knees coming up? Then you probably had good head position.
By the way, it's good to make a habit of paying deliberate attention to what you see while doing any skill, not just back tucks. The more you train your brain to notice and think about what you see, the easier it will be to have good aerial awareness and correct head position in almost any skill, and the bigger the skill, the more important this will be. The mental habits you build doing back tucks will come back to help (or haunt) you in layouts, fulls, double backs, giants, tsuks, you name it.