#1
whenever i try to harmonize i always sing flat when i sing by myself its easy but with anyone else its flat i tried doing it with songs but still i end up singing flat is there anyway to practice to where you just ignore the singer and sing what your trying to sing so that the end result is both of you harmonizing?
#2
And you are a guitarist, I assume? If you are, I don't know what else is there to do besides practicing again and again or maybe getting lessons; otherwise, if you have one available hand, cover one of your earholes with a finger - this way you can get better feedback from yourself even in a louder environment, some singers do this.
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#3
The problem is without the relative pitch you cant tell but you are probably always flat. It just doesn't seem like it when you are alone and that is why it is "easy." Practice scales with a piano (can be had for less than 40 on craigslist/ebay) and in weeks this will probably be well on its way to being fixed.