Ah! Position is almost undoubtedly the root of your problem. Instrumentalists who don't have to breathe to play tend to hunch over their instruments. There is no way on earth to get good breath support that way.

First ya gotta know here's how the breathing process works. Now, most folks think that the diaphragm is responsible for pushing air out of you. In fact, it has nothing to do with that! It is responsible for pulling air in. Your lungs, by the way, are actually sloped on the bottom and tilt upward toward the front, like this if you were facing to your right: / The diaphragm pulled your lungs down and forward as you breathe in. When you are hunched, it doesn't have room to go very far, see?

So, you hafta find a way to sit at the front of your chair with your feet in such a way that your thighs are sloping down toward your knees. This'll give you some breathing room. I know it's hard with a guitar, I play a bit myself, but it's worth it to sort your position out and get used to it as soon as possible.

Now that you are sitting like that, breathe in and visualise what's happening. Don't let your chest come up and out, that's a shallow breath and the chest rise/fall is not an indication that your lungs are filling up that much! You must breath such that you feel you're sucking air down toward your navel, and let your stomach expand. You can actually work on increasing your lung capacity this way, and eventually your stomach will bulge all the way up to the beginning of your sternum, out from under your ribs. My voice teacher looks like he's just swallowed a third-grader when he's all filled up!

Anyway, now you can just try singing. I don't know if you're tightening up your throat and tongue and fatiguing quickly therefore, but once you have the breathing down enough to turn your consciousness to something else a bit, start to focus on relaxing from your face right on down to the top of your chest. Don't be afraid if the sound you get is very different than what you've been producing and not what your looking for, just get the fundamental breath support down before monkeying with the rest of it! Noone can give you much advice without your physical presence from here on out, and even just a few lessons with a really good person can help more than you'd be willing to believe at first, so I'd think about it. Be picky though - a bad teacher can really screw you up, I speak from experience.

Let us know how it goes!

ciarili