Hello, I am having big troubles coming out with good melodies.

the vocal lines i come up with very flat. I do not know really what to do.
The guitarist in my band created a very nice instrumental, but when I put the vocal line over it, it sounded so flat. http://www.youtube.com/watch?v=OCE1FBRyw3Q

I am so depressed because I cannot come out with a decent vocal melody.

I would be so glad if you guys can give me feedback and point out my mistakes in making vocal melodies
i'd say that it probably has to do moreso with vowel shapes and "sitting" on notes instead of singing through them that are making you sound flat.
While you don't always have to do it, in general the accented syllable should fall on a down beat, as would the most important word of a phrase.

Forming vowels in the back of the throat rather than forward creates a flattening sound (this sounds weird, but visualize the vowel forming behind your front teeth). Are you a native English speaker? Your pronounciation seems a bit accented.

Lastly, in a string of related phrases through a verse, you should vary the notes. There is a recurring motif of

note note note 1step up note back to note.

This is a very tough thing, I wish you luck.