> it's so slow and frustrating that the melody in my head just disappears.
Use a microphone to record yourself humming the melody first, so you can get the original back if you forget it.
Separately, I'll mention that imitone (https://imitone.com) works surprisingly well. The transcriptions don't come out perfectly by any means, but they provide a base that you can then clean up.
I've tried that and it's so slow and frustrating that the melody in my head just disappears.