Higgins, it sounds to me as if it's not that you're not a writer, but that other things are getting in the way, and strangling your desire to write at birth. In a sense what you're experiencing is procrastination, even though you haven't really started. I blogged about procrastination and not-getting-on-with-what-you'd-sworn-you-most-wanted-to-do
here, and you may find that some of the causes I've suggested ring a bell with you. I may say you're not alone; that post has been one of my most responded to and most hit-on posts ever!
Another way of thinking of it is that what's holding you back is fear. You pour your all into it, and some of that all comes from some dark or secret places inside you, and at the very least you spent a year or five doing this and not talking to your spouse or decorating the stairs, and then you're walking out in public and saying, 'This is what I am, my most intimate, secret self (even if you'd swear it's pure fiction) and it's the best of me and the best I can do.' That's an incredibly naked thing to do, and sometimes I'm amazed that any of us can bring ourselves to do it.
There are other fears, of course - some of which I touched on in that blog post. It might help to see if any of them resonate with you.
I think Naomi's suggest of just writing can be a fantastic way to get yourself over all sorts of the blocks which I outlined. It's allowed to be rubbish (have a look at the NaNoWriMo website to get an idea of the reasons why it can work) so the pressure's off. Indeed, it's a necessary part of the process: as Anne Lamott says, shitty first drafts are what writers write, so they can then write good second drafts, and finally terrific third drafts (though there may be fourth, fifth, sixth...).
Emma