I can sing pretty much in tune a lot of melodies, but only when I play the same melody on guitar at the same time. When I hear the melody on a guitar I'm somehow able to follow it with my voice almost perfectly. However when I try to sing the same exact riff without support of guitar I can barely hit any notes. Does anyone had a similar experience and / or know how to overcome it ? Do you know if there are some specific excercise that address this problem ?