While I am entirely in favor of voice lessons and almost any other manner of musical training, I also believe there may be another possiblility to investigate as well. It might be that the instrumentalists who have attempted to back you are just too inflexible to work outside of set tempos. If you want to hear what it can sound like when a good voice is backed by musicians who can play "outside of the box", have a listen to Sinéad O'Connor's "Sean-Nos Nua" CD.