Unless you actually do something w/ those wonderful ideas in your head, they are nothing but possibilities and your gift is nothing but a presumption.
It's like assuming that you're good at martial arts and a great fighter because you've watched Bruce Lee movies. From your perspective, it's easy enough to win a fight isn't it? Do just like Bruce Lee - avoid getting punched and kick the other guy's butt.
As someone who's payed music ever since he was big enough to hold a guitar in his hands, studied, and listened to music all his life I'll also tell you this much - your "songs" no matter how awesome they are "in your head" are nothing but possibilities.
I have songs in my head all the time. And even w/ my experience and training, it still happens regularly that something which sounds great in my head fails miserably when I actually start working on it.
So your option is to give up the assumption that you are gifted for a second, and put yourself to the test. Unless you simply want to bask into the unverified certitude that you are actually gifted.
At that rate, you may as well tell me that you could kick Bruce Lee's ass. If it makes you happy to be so sure of that, go on and dream away. I'd call that wasting my life, but if it's enough for you...