Is this lithp permanent, is it caused by my biteplates on the backs of my front teeth?
If you are new to the bite plates then it may take you awhile to adjust to them. Your speech will go back to normal when you do. One why to make the process faster would be to practice talking or saying the words you have problems with. You can do this whenever you have free time. I would not let this upset you. Just talk a little slower and be more carefully with pronunciation until you are better at it with your bite plates in.