Hello! Yes, corner fireplaces are difficult to deal with. I know that the photos below are a little different than your house, but they can give you a good idea of how to achieve the look you want. First, move your TV up around 6". Next move the sconces flanking your mirror down about 6" to 1' so that they are centered with the mirror. I'm not sure of your furniture configuration, but I think that you should move your TV stand and TV over to the left so that it is centered with the wall, is that possible? Then flank your TV with things that are about equal height or maybe 1' higher, but that's it. Don't go too high or else your TV will look short again. Having a few items that are in the middle will help tie in the mirror which is placed the highest. Good luck!

Built ins for what? Here are some room layouts for corner fireplaces.