More excellent advice above, but here's another approach, an ancient one if we believe the scholars: I have been singing ballads and songs for forty-odd years and had a rest from the big ballads for about 20 years but in the last couple of years have come back to singing them in a new way (to me). I select a tune and refrain I like and then from my memory of listening to lots of different versions over the years piece together a version without referring back to any one version, keeping it concise. I also remake the songs each time I sing them. So far it has worked on The Cruel Mother and The Two Sisters. About a year ago I did a workshop with a Norwegian singer and we swapped versions of Tvo Seostres, a moving experience.