Lyrics are often the writer's main frustration. If you're a good musician but can't put a sentence together, it can be particularly irritating. Especially when a teenager with three chords and a pretty face kills the audience with a song about his pet rabbit. Dylan says that the biggest struggle songwriters have is to get in the way of their songs.
Some of the best compositions are done in a hurry, while one self-edits less. If you write a line and judge it immediately, you're conditioning yourself to write less. Your mind doesn't want to do things that are painful for you. Try physically imitating a vacuum for 5 minutes and then write on it for 30 seconds.
Cut out interesting images from magazines. Then rearrange them and write a story based on your layout. Sometimes, all it takes to write a song is to listen to a new piece of music by a new artist and be instantly inspired. However, if every time you sit down with your instrument, you end up playing old songs or looking for new versions instead of writing the songs you intended to make, you're stealing valuable time to finish new songs.
I can sit with an instrument and have feelings flowing through my fingers at times, but trying to write lyrics is a frustrating process of writing and rewriting and rewriting until I don't hate it (but I never get to a point where I like it). Bass guitar, cello, a lot of musicians write a lot of good songs for a bass line, it can be done pretty easily if that's your thing.