Writting for the sake of writting will usually find you frustrated, the more you try to think of something the further away it seems.
it depends on why you write songs i suppose. I agree with a previous poster in that for the song to reach a large audience there has to be a format that is commercially however i don't think this is always the case.
The greatest songs (not necesarily the most successful) but the greates nonetheless are those with meaning... A song that when listened to the listener can say wow I know exactly what the writter was feeling when he/she wrote that. The beauty of music is the power it has to affect the human mind and spirit. Some songs can whack a smile on your face within the first bar others can grab your heart and pull it out of your A***
Listen to your heart when you write, ask yourself what you feel about the subject, be it a person, an event, a place, a relationship whatever...
There was a 1 year gap between my very first song and my second. I wrote the first having learned only 3 chords on the guitar lol but it worked and it made me want to learn more.
Don't be too hard on yourself and NEVER throw lyrics away!!! It might sound s*** to you now but in a few months time you';ll be writting another song and suddenly you'll think hey that part of that other song would fit in great... it happens all the time... Keep everything you write, every idea every word is your creation!
And most important NEVER NEVER give up... if you're stuck, put it away and take a break, it will come to you.