Try copying a tune you really like. From just how you remember it - don't actually listen to it and try to transcribe. You may be surprised. For me, it was Fever Ray, "If I Had a Heart" - the Vikings theme song. Listening to it later, I realize I was only fairly close to the main phrase I was trying to capture - but I like mine better.
There's a whole field of Psycho-Acoustics - hearing what we WANT to hear rather than what's actually playing. And I learned a lot in the process (was my first attempt at anything musical).