Thank you for this!

Autumn • Poet. Addicted to coffee and cuddles.
I so needed to find a group like this. My boyfriend has an 8 year old daughter from a previous relationship, and lately, I have been experiencing a great deal of confusion and pain over the situation. His daughter is beautiful, and we have gotten along splendidly since the time that we met. Still, I find myself growing very bitter with the situation. I become frustrated that she is not my daughter due to the fact that he must continually keep in contact with the actual mother of his child. I feel pain knowing that he will always have this bond with another woman because of his daughter. It tries my patience when she misbehaves (which is more often than my boyfriend would like to admit. He spoils her and insists that she is a well behaved child, and for the most part she is. But she is smart and knows how to take advantage of the situations that she is in, even with her father) but I feel completely unable to do anything about it because I am not her mother.
We have been talking about transitioning things so that his daughter lives with us in my apartment. I am frightened by the responsibility and pressure of being a step parent. 
How have you handled the transitioning? Do you have any advise?