My voice sucks, too, which was very dissapointing to me when I first heard it played back on a recorder. All I wanted to do was play and sing my favorite songs, Nirvana included. Was that too much to ask?
Fast-forward ten years: I still play and still try to sing, and I'm getting better. Try to emulate the sound of the chord with your voice. Don't try to sing too high if your voice is naturally low. If you just can't make a particular song sound right, try a different one.
Also, work on your instrumental playing, so you don't have to sing!
Don't give up on your music, man, it can get you through rough times.
DC