Rrich presence of a father in his child's life  

I am very sad and hard to understand him, it has been already 3 months that I got my divorce. and the most difficult part is seeing our son in fatherless situation. his father out there having fun with his mistress, and do not think about his son.  our son really needs him , and when he calls him to come  and take him for some activities , his father refuses because he is busy with her, but doesn't he realize that he breaks our son's heart, doesn't realize that he is missing out on the best part of his son's life, , when i see our son sad , I mean deeply sad, I just want to die, I regret , and tell myself may be divorce was not right thing to do , but he was with his mistress and his mistress did not leave us alone, we did not have chance to work on our marriage I tried very hard, but this woman 's heart is made os stone, did not understand that I just needed my husband with us   for a few more years, may be our 11 years old son could understand this situation better.I wish divorce 360 could send him some kind of informations about  the need of a child, I know nothing can can replace the happy family that we had , and how could I make my son understand that the family part of our life is dead. please help me, tell me something that could make my job easier, presently I am a mother and father , and I am doing more than 100% , I bought our house from him not to move somewhere else smaller or my child loses his friends I absorbed all the debts I am doing all sort of activities for our son , but still I see that can not replace the absence of his father. I do not have any family here and his family they do not talk to me we are very isolated, and do not have many friends , but I often invite my son's friends and try to make life more pleasant for him. you have no idea how difficult it is to see my son in this situation.