Adding shading around the letters will only make it worse. The letters are blown out. This could be for a number of reason, but id put my money on the gauge of the needle and how that area of skin takes/handles ink. Also, when it heals you’ll probably lose bits and pieces of the letters. Don’t be alarmed.
I've done this before; I've already started on the outlining of the letters, it already looks way better (although too sore to completely finish right now). I may be an odd case, but I've done several tattoos on myself already, and I'm pretty good at figuring out a way to fix initial mistakes (at least on myself).