I think a lot of writers think like that. The more you think about your story, the less it becomes exciting and original in your mind. But, to your readers, it will be something new and exciting. Your story will still be 100% your thing, because it all led from your own creativity. Even with common tropes, your story can still be new and fresh.
As for what I do when writer's block hit me, I simply stop writing. I relax myself, not stressing about any of my projects. Fortunately, I don't write for a living, so there's nothing to lose by doing so. If my writer's block is of one of my project, I sometimes do another project or short story instead. Or, sometimes, if I just don't have the mood to write but the idea in my mind keeps flooding, I'll write those ideas down, linking one another to form a coherent sequence that can be my reference when writing in the future. All in all, I'll try my best not to stress myself--maybe while I read some other stuff, or write some other things.