Why does holding a guitar while singing make me sing worse and how do I fix that?

I've noticed that I not only sing worse when actually playing guitar, but I also sing worse when I just hold it. I'm not sure why. <Maybe it's because I'm automatically slouched over the guitar when I play it? I also usually cross my legs because if I don't the guitar just isn't high enough/close enough to my body. And the leg crossing makes my body less relaxed I guess. Is this stuff a common problem? What can I do against it?