This is just me venting...and looking for advice from ppl in similar situation

Sooo, im engaged...to the love of my life. But she was married before, and has a son from that marriage. The boy is 3, and i dont look at him as my fiancees son, i look at him as if hes our son...as if, im his dad. My fiancee was married to a real jerk. He cheated on her constantly, abused her emotionally, and sometimes physically, and just all around completely shattered her heart. And then on top of it, hes not a great dad. He comes up with every excuse in the world as to why he cant see his son when he is supposed to. My finacee, even tho she doesnt want him to see him, because hes got two other sons from a previous marriage he was in, who are not very good role models, wants his father to see him so he can atleast grow up knowing his dad. This is where my feelings come in. My finacee, her son and i live together, i guess u can say were a family. I take care of both of them, and give them both all i can give, all the love i can give, all the care i can give, i love them both more than anything. So it bothers me a lot when my fiancee gets very hurt and upset when her ex makes dumb decisions or doesnt want to see his son. And i feel that way because, i feel, "I wish she wouldnt care about what he does or says, cuz Gray (our son) has me"...but i know thats not the case...at all. I sometimes want to tell her, look, why do u care what he does or says, im here now, im helping to raise him? And of course i cant. Its just one of those in our relationship that no matter what i do, or say, or how happy i make my fiancee, or how much i make her smile, or laugh, or how great things are between us, it can all be negated but whatever her ex says or does. I just feel so helpless. She cries, and theres nothing i can do, or say that will help, because there isnt, and that makes me feel like shit. We can have a great day together, but if at the end of the day he calls and says i cant see my son, or says anything else that effects his son, theres goes her great day. I just feel so terrible, and helpless when all of my effort to comfort her and be supportive of her, and take care of her son, and make sure hes got a father figure in his life, can be washed away instantly by the decisions or actions of another man. I donno what else to say...i almost feel like im not enough...that ill never be enough, because no matter how much i give to her son, no matter how much i teach him, and play with him, and love him, im not his dad. And being in this situation really does make me feel like ill never be enough. I dont have an ego at all...but feeling this way is almost, u could say, "a blow to me ego." Thoughts and comments?