Hi how can i fix this ugly shadow glitch???

[true hearth]

I made a simple locker model and export it to minecraft resource pack but unfortunately this shadow glitch appear and i don't know what i should do to fix it

[toxic tapir]

!faq java dark

[scenic crag]

(Result for "java-block-dark")
If your models have black spots try these steps:
Opaque rendered block will have black spots if:

• They are connected to a other block in the same render way
• If elements are outside the 16x16x16 area
  There are some ways to remove the black spots:
• Disable AO (Ambient Occlusion)
• Disable shading
• Moving the black elements or rotate them
• Disable smooth lighting
• Use OptiFine (helps a ton with the crappy shading of mc)
• Centering the origin point to the element (or moving the origin point somewhere else sometimes works too)
• Setting proper cullfaceing
  Sometimes one of these steps helps, sometimes multiple of them help, and sometimes it is unchangeable.
  If nothing works to fix it, then you should use a transparent/translucent rendered block instead, or place it next to a transparent/translucent rendered block.