Try changing how you approach writing. Write the melody first, then the words. It's what I find successful. For years I never thought too much about writing. I just wrote, words or music first as the random notion took me. Many times I found it similar to yourself. In writing melody first I set a meter the written word would use. By training myself to work that way around I was less precious about the lyrics, I was less attached, less invested in the lyrics being as written. The fact is they never were written in stone, but sometimes it felt just like that. By writing the melody, taking the rhythm and using it for the meter of the lyrics I knew what i had to work to right from the start.
Make sense?
To be honest, listeners tend to hear melody well before they follow lyrics despite the fact that genre affects whether lyrics or melody are given more weight. So in my mind melody wins out. I know I can write no matter the rhythm/meter, the difference is only in that sense of attachment.
It's a good thing to get used to, not being too precious with your songs. One thing worth noting when digging out that edit scalpel, the original still exists no matter the changes. Work is not wasted, you either use it and learn, or don't use it and learn. Going back and forward making tweaks, too afraid to carve it up, dissect what works and what doesn't etc, now that is a waste of time. It must be said, working with the critique boards helps with that. If you find yourself getting defensive, it's usually that over protective, overly precious monster whispering in your ear. It's about the song when it comes to critique, not the writer. Sure the writer and the critiquer both learn, both explore and exercise their songwriting tools, but the focus is what is good for the song, what ideas would help or ruin it. If you remain too precious of lyrics or melody or mix it acts as a block. your mind is not open. This is no doubt where you are stuck, even though you may be acting as both critiquer and writer on your own songs.
I also recommend doing a lot of critique of the works of other writers. To be honest I think you learn more giving critique than receiving. You get to exercise almost all songwriting skills, improve your observation and analysis, and solution finding, without being in the slightest bit precious of the work. It's not your song, so the decisions are not yours and you are not invested in any one part being any special way.
Both help you gain the needed objectivity, an objectivity that is easier to turn on and off, allowing you to get in the emotion of the song, remain connected, but to drop into a more objective role when needed.
I hope this helps.
Cheers
John