This song's for the one I loved, at least that's what I thought.
My intent is to raise doubt about whether I love "the one", but I think "at least that's what I thought" could refer to the whole first half "This song's for the one I loved", i.e. I thought the song was for the one I loved.
Which interpretation is correct? If the second, what would be a proper way of saying this without really shortening the sentence?