I am not a teacher, but I am a native speaker.
Try not to get upset over not understanding slang completely. Regular English changes very slowly, but slang can come and go in trends. Slang is often connected with movies, music and television, so slang trends can come into use and fall out quite rapidly.
Just try to experience every source of native speaking you can (movies, books, conversations with native speakers, etc.) and just figure out what each slang phrase you have never seen means. After a while, you will understand a lot of it. Understanding slang is not just about listening skills. Because the meaning is not often apparent, it will oftentimes take some research to understand the meaning.