I Moved Out of Flat This Morning

... I did not have much happy memories in this flat because my room mate is a jerk.

"In" is correct.  

Do not use "from."

"From" designates a starting point.  

"In" describes a period of time.  

In

http://www.merriam-webster.com/dictionary/in  

From

http://www.merriam-webster.com/dictionary/from

I Left My Flat This Morning.

I lived in this flat for 11 months, and now it's time to give back the keys and return to China. I do not have many happy memories from this flat because my roommate is a jerk, but I will miss everything here. Regards...and good luck! :D

I Moved Out of My Flat This Morning

I have been living in this flat for eleven months, and now it's time to turn back the keys and move to China. I did not have many happy memories in this flat because my room mate is a jerk, but I do miss everything else here.

I Moved Out of My Flat This Morning

Sorry, missed that in my original short correction.