How can I add those white lines inside the letters

Typically this is done in a vector drawing application and the inner highlights are added manually:

It’s also possible in Adobe Illustrator, to use multiple fills and a clipping mask….

_{The base shape (N) is used as a clipping mask on the existing N with multiple fills.}

If I were forced to use Photoshop and could not use the Pen Tool to draw the highlights manually, then I’d use selections of the base shape and a mask on a separate highlight layer.

Drawing the highlights with the Pen Tool in photoshop would provide more control overall, and really that’s the method I’d suggest.

You could also spend an inordinate amount of time trying to figure out what exact Bevel & Emboss settings, along with the Contour may result in a properly positioned highlight. That’s more a trial and error exploration.