This is a great topic, and is my first reply on this forum as a new user (just purchased BIAB yesterday and just starting to fumble around with it).
I can personally attest to the advice of "don't worry so much about what you're writing - just write, dammit".
I have been playing guitar for 50 years or so, and have picked up several other instruments along the way.
But every time I sat down to try and write, I would get stuck. To me, my writing was never "just right" and I would criticize myself into analysis paralysis - then just give up.
Then, about 10 years ago, I decided to try again.
I started with lyrics. I went to RhymeZone.com, and just began putting song ideas together, often just starting with a title.
Once I had my verses, choruses and bridges spelled out, I put music to them - kinda like the way Elton John does it.
I literally did not care if the finished song sounded good or not.
In fact, one song I considered an absolute throwaway, took me 15 minutes to write and another hour to record - and it is still my wife's favorite song by me. Go figure. Here it is:
https://soundclick.com/share.cfm?id=7362038Anyway, over the next while I ended up writing and recording 20+ songs, when I was totally convinced I couldn't even write ONE!!
I think some are pretty good, some are terrible - but I don't care.
I just sat down and wrote.