writer's blocked by concepts

[busker]

I've been working on a collection of poems for the past 4 years and I've reached a point where poems come to me much more slowly than they did when I first started the collection. Before I remember writing 2-3 poems every month, but now I'm lucky if I write 1 in a month.

I feel like a big contributor to this writer's block is that since I am now around half way into the collection, I kinda have this flow of ideas mapped out and it is something I am trying to follow, religiously. So, recently I've only been looking for images that fit these ideas, instead of finding the idea in an image. is this a constructive way to go about writing poetry?

Some might say that I should forget about the collection and just write, but I am simply stubborn about it to the point that I will not write a poem unless it'll go into the collection. How should I reflect honestly on this mindset? What do you guys think about centering everything you write around a highly conceptual collection and how is this ever done? Is this the right way to go about it? Do high-concept collections just happen, or are they sometimes planned like how I'm doing?

I don’t think Paradise Lost or the Divine Comedy happened, they were planned and executed painstakingly. It is not an easy task. Keats failed to do it with Hyperion. 
While a collection is not as difficult as writing a long narrative poem, it’s still harder than writing a bunch of spontaneous lyrics.
What’s your goal here? How many poems have you planned and where are you at?

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I tried to write a long narrative poke once, a long time ago. I couldn’t finish, even though I’d started