Just like playing the guitar, songwriting is a learning process. It's just like looking back at how well you played guitar a year ago... Everyday that you sit and write songs or just write in general you grow a little bit. Granted, some people just can't write songs. Just like guitar and anything else... it follows nicely with the statement "if it were easy everyone would be doing it."
The biggest difference between the stuff of mine that I don't like and the stuff that I do like is the maturity of it. The songs that I wrote 3 years ago are very basic, good in their own right... but I don't ever play them for people becuase I feel like they don't show what I am really capable of as a songwriter. Since that time I have absorbed so much thorugh life experiences and through music that my writing abilites from then to now are so much more broadened that it's really amazing. Writing is one of those things that takes time... My writing skills have matured along with me. Through all the life experiences that I have along the way... each and every day... I become a slightly better songwriter...
I am, however, my toughest critic and I think that's the way it should be. Stay humble. If there is something that you don't like about a song and everyone tells you should leave it, becasue they like it.... Chances are they're lying to you and you should change it.
In the end we all have growing pains in anything we do. Writing songs is no exception to the rule... It doesn't happen overnight.