What would you do .... ?

My Bf and I live together along with his 5year old son. His previous relationship lasted about 8yrs on and off and ours is no where near eight years. Well his sons mother and I dont seem to get along much. We both know she still wants to be with him, and that makes me unconfortable, knowing their history and that they are practically family to one another. She is very sad, because she feels I am trying to take her spot in her sons life from her. And that is not my intention at all, I love my Bf's son as he were my own but I know my role, and I am not his mommy. She is also very sad because she feels she has lost her friend which is my Bf (her ex). To be honest I know the feeling but I dont feel comfortable them being so close for my own insecurities and for the fact that I know that she still loves him and that he still has "love" for her as a friend and mother of his child, meaning he cares alot for her seeing as how she was a big part of his life for a long while. Now I know I must be strong and trustful of my Bf and all and I know he doesnt want her to be sad all the time, it depresses her, him as well as their son. & they want friendship and closure but I just dont know if I can deal with the fact that his babymother is a constant figure in his life always. Ive nvr dealt with it, and it is very difficult for me!