The most difficult part for me comes when I can't conceive something worth writing about, or when I don't care enough about what I will write. So, for me, the most difficult part comes when I lack the motivation to feel strongly enough to write. It's more apathy and lack of inspiration than not knowing where to start.
I think this is why I enjoy writing songs for other people so much. I can read a few lines someone writes to me in an e-mail and get a world of empathy going for that person. Then I can channel my own imagined picture of that person's life into a song, because I truly believe the person is interesting and worthy. When I look at my own life, I have trouble glamorizing it to the point where a song flows out of me.
I don't think songs need to be written about ourselves all the time, and I like change of perspective in songwriting. For example, Mark Knopfler wrote a song called, "Boom Like That," which tells the story of Ray Kroc who got McDonald's restaurants to become an establishment of fast food. For Knopfler to do that, he had to empathize with Kroc, know the historical context, and understand Kroc's nature.
So, instead of writing selfish poems about myself my whole life, I want to branch out and write about other people. My writer's block is inspiration and motivation, and the rest comes natural. So, do you have some inspiration to tell me?