Learn Morse code
The Koch method: characters at full speed from the very first lesson, with the silence between them stretched to whatever pace you can follow. A new character arrives only when you are copying the ones you have at 90%.
K M
Why the characters never slow down
Sent slowly, a letter is a sequence you count — and counting is the habit that stops people at ten words a minute for years, because there is no speed at which counting keeps up. Sent at full speed with room between them, a letter is one sound you recognise. That is the whole method: the character speed stays high and the gaps carry the difficulty.
So start with the overall speed low — 8 or 10 words a minute is normal — and leave the character speed at 20 or above. As copying gets comfortable, close the gap rather than speeding the characters up.