Try humming along with the melody you already have and launch off of that to see what you come up with. A good melody in a song isn't the fact that each section of the melody is awesome, but that it flows in a pleasing continum. Sometimes if you just let things take their natural movement, something really good with come out of it.
The important part melody plays in music is feeling. Melody ultimately sets the mood for the song. It has to, because to musicians and non-musicians alike it's the most paladible thing to the ear. Everyone connects with the melody. So relax and try to feel the song you want to write, and play off that feeling. Alot of the times when you do this, the melody (and other ideas) will just fall into your lap.
Everyone at one point or another has felt the same way you do now. I know I've written songs and gone back and listened to them and gone, "what the heck was I thinking". And it gets tossed. I've only kept a small fraction of the songs I've written due to this fact. It's just a process we all go through. ;)
"My whole life is a dark room...ONE BIG DARK ROOM" - a.f.i.