Pro advice:
Imagine if you had some yellow paint and you added some blue to it, it would turn green. That’s because the blue is interacting with the existing yellow colour.
When colouring your hair, you have to consider your existing hair colour (dyed or natural) and the colour you want to achieve before you choose the correct (box) colour to apply.
As others have said, their box dyes often end up looking darker because they’re being added to hair that isn’t neutral palest blonde.
When you make a cup of tea, if you leave the tea bag in for a long time, the water goes darker. So brown tint on top of brown hair is bound to look darker because it’s being saturated with lots of extra brown molecules.
If you want to hide your minimal grey hairs, you need to choose a colour that is almost identical to your existing hair colour, not a darker shade. You want the colour to blend in not deepen it.
However, I do think your hair is gorgeous as it is and I’m sorry that you’re struggling with body dysmorphia. It’s very debilitating and I hope you reach out for help for it at some point. You deserve to feel happy. 😊