Read up on the Koch method. To be exact: Koch method with Farnsworth timing. You'll have to keep the letters in a certain speed so your brain does not have enough time to decode dots and dashes. So only the length between the letters is extended to lower the difficulty. The idea is that the lowest unit of comprehension are letters. After training you'll hear a -.- and don't even think of it as dots and dashes, it's just a K obviously.

You start just distinguishing two letters (usually K and M). I would recommend using a trainer app that gives you a string of letters by sound, and to write them down on paper. Then you can check how many you got correct. Do not try to replay letters multiple times, just skip it and move on. If your result is better than 95% or so you can continue and introduce a third letter, and so on.

There is a certain order of letters considered more or less canonical for use in the Koch method which you can look up.

Edit: Back then I learnt it with https://lcwo.net/, it's distraction-free and quite nice.