I was walking down the street
in the afternoon
and I saw her straight in front of me
With free-flowing hair
and deep peaceful eyes
the most beautiful girl that I had ever seen
and she was smiling at me
with the sweetest radiance in her face
and I looked and smiled back at her
and realized, that she wasn't really there at all...
more a shadow, or my imagination
and I knew that I missed you more than anything
This is very pretty! The twist at the end makes it a piece worth reading and the verses don't flow in a traditional way, but you made that work very nicely regardless. I enjoyed the imagery most of all and think you have room to add even more of it so that the lines don't seem commonplace. Don't stick to the most basic way to put things. You can get more creative and still get the same point across.
As for specific feedback, I want to focus on the line: "And I realized that she wasn't really there at all...". This is actually both my favorite and least favorite verse. I like that you suddenly shift from realism to this vagueness, however, the only real problem I found in this piece was that you jump into that realization without any introduction as to why you suddenly noticed that the girl was imaginary. Did she do something unrealistic? I think if you mentioned that transition it would bring the whole thing together.
Altogether I thought this was very good and would like to read more of your poems in the future.