Hi Lily, lovely fiddler (all female fiddlers starting with an L are lovely) - it's only a problem of training, best done together with others. My experience: While I got music lessons with a lot of theory and sheet music I learned tunes by ear with the boy scouts. We sang together, and when we were all in unisono we had done well.
When I intend to learn a song by ear I involuntarily hear the main intervals and steps, too. That's my classical training. So it is unimportant which instrument I use, or which key.
I often have to do this when I'm checking the DT for tunes, and I still remember that "John Kanaka" proved to be difficult at first for a certain interval ... So I started again and again, till I had mastered it.