I'm guessing someone may have already said this, but - since you've had such close contact with her, and then with her pillow when she passed on, you've been absorbing her pheromones, skin oils, shampoo and soap smells, perfume, etc. That stuff doesn't just disappear for a very long time, it'd get into your skin no matter how many showers/baths, even time away isn't going to fully separate you from her smell. You might have gotten used to it, but I imagine someone else who hasn't spent time with you could smell it.

At the same time, yeah, your smell was getting into to pillow, the same way it got into her skin before she passed.

Think about that. You spent enough time together where neither of you even smells the same anymore. That smell you go on and on about in your head? That isn't just her smell. That smell is the both of you. That smell is together. You could throw the pillow away and you'd continue to have that smell - that smell that isn't just her and isn't just you, it's your life together.

It might fade. It might get harder to smell (though I believe there are things you can do to fix that). But really it's harder to smell because she was part of you for a