I've been long time in United States for a long time.

I came to Washington, DC in last September. I am really happy to have spent my time here and to have experienced many things which are I can't experience them in Turkey. I wasn't able to improve my English as I wanted to, but I tried my best and that was the point. I think the most important part of learning a new language is learning their culture and their daily life. If you can feel yourself just like them one day, it's done. You did it. I had (or: There were) some barriers to my learning way but as I said, I tried and I'm happy :) I hope I'll come back Washington one day and I'll walk again down/through these beautiful alleys and parks. See you again Guys !!

Good job! The second-to last sentence is a little tricky because we usually say "walk down an alley" and "walk through a park." It would probably be safest to say "visit" or "see."