1) This sucks.
2) I stole this melody from another song – I just don’t know which one.
3) This is a stupid idea.
4) This is just like that other song I wrote last month.
5) This won’t amount to anything.
6) This lyric is sappy.
7) It’s too much of a ripoff of this other song I like.
8) There must be a million songs with this title.
9) I should really get this room/apartment/house organized right now!
10) I suck.
These are all questions that may need to be addressed at some point in the writing process (except possibly number 10, which may require a more therapeutic approach), but don’t let them stop you from writing.