This is more like a poem then a riddle.
It could be
At first I heard it and thought i could soar like a bird!
At start you saw something exciting in the dream thats what "i heard it" is referring to. And then you decided to fly into the dream like a bird.
And oh what i heard!
Suddenly you saw someone in the dream
I’ll zoom so fast, my image will be blurred.
Then decided to see him clearly(zoom) but its blurred. Well all dreams are a blurred image.
And now I see it, it’s flat as can be!
Then suddenly wake up from a dream and sorted out that was not real. May be flat here refers to coming to the actual flat world.
Oh woe is me.
And how lonely, just one single tree.
After waking up from dream you found yourself all alone again. a single tree represents loneliness.