Myself unassertive person

Sometimes I fell myself very unassertive person. I also feel that I really nice person and softly person. When my friend know my behavior like that they always get some benefits from me. Just like this semester I have some friend, she also my group member. She likes the person that does not have a friend in that class. So poor when know she fate on this semester. After she become my group project member, she likely do not want to do that project. She really lazy to seek or give some idea about that project. When we ask her to do some work, she always answered that question with the word "Sorry, I do not know to do this work" or "Sorry, I really can not find that information". Oh my God, why I very nicely to invited she joined we in this group project. Finally, I become angry and be so assertive to her. I really do not know whether I doing this right or not. I can not control my temple that time, I angry to her and telling to her about our feeling. On that time she really regret about her behavior toward all of us. After all that happen, we use the communicate assertively. This is because to make sure that she will doing well in our project. Maybe this incident gives we lesson from our mistaked when do the group project.