Increase the speed of the characters with longer spaces between. This is termed Farnsworth timing, sometimes shown as a ratio - 20/15 would be 20 wpm characters with 15 wpm actual speed. While it is not proper Morse, it is considered by many to be an effective learning tool. It can be combined with the Koch sequence. If you find yourself still counting character elements (dots and dashes) at 20, increase it to 25. The speed has to be fast enough that you can't count the elements. Eventually you will hear the "sound", rather than a pattern that you have to mentally visualize before you recognize it. Then, after you have learned a few letters, decrease the length of the spaces until you can do those at standard timing. That is, you work up from 20/5 to 20/10, 20/15 and eventually 20/20. At this point, add another letter or two and go back to Farnsworth timing so you can copy them all. This "combined method" takes a lot of time, but it is a lot less frustrating than listening to something you can't follow because it is too fast. If you don't have something which can do both Koch and Farnsworth, I recommend G4FON's trainer for a PC or Wolphi's Morse Trainer for Android phones.