crazy, i had a very similar experience. i am also relatively new at singing and such, and for my first year i wrote what i thought were decent tunes, and to this day i think they still are....and then i reached a point where i couldnt do it. and this "writers block" lasted nearly a year. i was doing ambient electronica stuff. and i just could NOT write any more good songs, lyrics were all wonky and i would have one good musical idea and then had no idea how to build on it. i understand your frustration. what i ended up doing was changing it up. i started with a different kind of genre, i went from ambient, to piano and vocals only. the occasional simple background guitar riff and bass just to fill it up. it was a big change, and i was back at spitting out songs like i used to. and now these days i've kinda combined the two genres.
now i'm not saying go change the genre you are trying to make, but maybe make it in a different way than you are used to. try different methods. think of a melody before you think of lyrics. just do the opposite of what you are used to doing? i don't know. i hope i helped a bit. and if not, i hope i gave you some comfort in knowing that you are NOT alone.