I have no problem writing songs. Chord sequences, melodies, arrangements - and especially lyrics - come very easily to me. Of course, that is no indication of the quality of those songs, but that discussion belongs on BTR.
The only reason I say this is because on this thread I see a lot of (what I interpret to be) low self esteem and crippling self-deprecation.
I think a lot of this stems from the self-inflicted pressure to be ‘original’, I see this so often in my studio with bands and artists.
“Our” music is Western Art Music. For all intents and purposes, there is no other type. Because of the way WAM is structured, there are no original chord sequences left, and probably no original melodies. So if you’re worried that someone else may have already ‘done’ what you have written, you’re right. But it’s time to get over that fact. It doesn’t stop people like McCartney, Jack Blades and countless others from writing, so it shouldn’t stop you.
With lyrics, you have a greater chance of being original, but again, I see many people crippling themselves through the belief that whatever they write should be show-stopping. Nothing could be further than the truth. Just write it down and stop worrying about it. Look at all the bog standard lyrics that accompany most commercial songs. “Da doo ron ron” I mean, even a six year old child could do that.
As for subjects to write about…
You can write about millions of things, literally anything. But this fact in itself can be daunting, I know. So try narrowing down your options. Try this: watch the news and pick the story that you can relate to the most through personal experience. Then write your lyrics. Honestly, it’s not difficult. Just try not to get too specific. The more ambiguous you are about the subject, the more people will be able to relate to it. And here is something you will discover - when people ask you what the song is about, and you explain the lyrics to them, they are invariably very impressed.
Here’s the kicker - you strive so hard to be original, but if you truly achieved that, the chances are that no-one would like your song. The reason people like songs is because they have a melody/sound/chord structure that is reminiscent of one they already like. They may not be able to name that song (if they can, the chances are you have got a little too close to the original) but the familiarity brings them a warm fuzzy feeling.
Originality, by and large, comes from performance. That’s why songs are covered so much. Same song - new performer. People never grow tired of that, so the desire to be original in your songwriting is demonstrably a misplaced one. (I’m referring to mainstream here - there are obvious exceptions such as Cage, Glass, Messiaen, even our own Emma to a degree).
So release those shackles - and get writing!
I really think it’s a combination of left brain stream of consciousness (words), and right brain ideas and concepts + emotions. They have to be in sync or it’s a mess. Making it work with a musical idea can be challenging if it doesn’t come along with a melody or rhythm. Sometimes imagining the performing of this new piece on a stage can help. “Theater of the Mind” and all that fun stuff. 
