Well, as you probably have found out... Not all types of lyrics will work with all types of music... For example, if you were to write a heavy metal rock song, country lyrics would not work out well with that. So you need to try to write the lyrics with the same feeling you wrote the music. Oh, always remember that just because some lyrics don't fit in with a song you wrote, that doesn't mean you can't use those lyrics later in another song.
Are you writing a singing melody? As in, are you writing some notes for your voice to sing (or are you singing random notes).

After you've written a singing melody, try to assign each note to a syllable of your lyrics so say if your line was:
I need some hot snatch
And your singing melody was:

You would sing the "I" as a C note, sing "need" as a D note and so on.

There's more to it than that (I recommend learning theory, analysing songs, borrowing a melody writing book and learning counterpoint in that order), but that's the sheer basics.
[U]        | |                     [/U]
[U]        |/     .-.              [/U]
[U]       /|_     `-’       |      [/U]
[U]      //| \      |       |      [/U]
[U]     | \|_ |     |     .-|      [/U]
      *-|-*    (_)     `-’
^ +1 ^

Have you tried writing the lyrics and then trying to come with your guitar part after you've got lyrics..?