FWIW.
When I think about my entire voice, I think of a magic elevator. The space in the elevator shaft never changes. The lowest notes are always open to resonate in the very top of my head. The top notes are always open to resonate in the 'chest' space. The elevator is magic because I NEVER MOVE IT. It moves itself up and down the shaft. But I never close off any of the space. And I think about low notes being able to resonate in my head, and high notes resonating in my chest, without pushing at all on either end.
That's not all there is to it, but I find this picture for LETTING the voice shift registers (instead of me shifting it) helpful for allowing the muscles to balance each other out. But also, as Susan said, you're young and this takes time.