Agree with the mix of Always and Tom. Stop trying as this is just one of those rare things that forcing it = zero result. I have music in my head but can't get it out so I don't try. I can write and when it comes I think it flows but then it stops, just like turning off the tap. It'll come back though.
Musicians I've known can't write words and so collaborations happen. This may be a way to begin for you. I hear apiece of music and I have a problem stopping the words coming, maybe you're the same with music.
Write things down though, carry a little book with you always - this allows thoughts, ideas, insights, sights and sounds etc to not be forgotten, and to be revisited at a later time. You're frustrated, write down the list of everything you feel until the words have run dry. Apply a mind map to them all.
And don't worry if you can't, we cannot all be "songwriters", but together we can write great songs. Send me a some music if you want. Post some music and ask everyone to submit lyrics. Work with what you have, not at what you don't.