Songwriting - Subject, direction

I've been practicing songwriting for a couple of years, mostly composing music. Recently I started focusing more on my lyrics. I think my lyrics improved when I started writing them like a letter to someone, just like I write this message. But I've got at least two problems with my lyrics. The first problem is I don't know what to write. The second problem is when I've written a first line that I like and it fits well with the music, I don't know where to take the lyrics in the second line and so on. Usually I come up with something that I like and I'm half-way into the first verse. Now I'm completely stuck. It's like trying to start a car without gasoline knowing that there is some left in the engine. How can I come up with stuff to write and how can I understand the direction of the lyrics?