Why do I miss my terrible ex?

I was with my ex for almost 15 years, we have 5 children together. It was a long struggle of a life together, filled with cheating, lying, verbal abuse...he'll tell me all the time that I need to remember that we did have some good times and that I will admit. I do not regret any of my children at all and know that without our relationship I wouldn't have them and without all the misery he put me through I wouldn't have become the strong person I am now. I'm trying to set a good example for my kids and show them that a person is supposed to be loved unconditionally and not mistreated or taken for granted, I have a new boyfriend and we have an awesome relationship, he loves me unconditionally, it's so different to me to have someone love me heart, body and soul and he loves and adores the kids as well. I feel blessed to have such a good man introduced into my life, someone who has told me he knows that loving and being with me means taking on a relationship with my children as well and even lending a hand with them and that he's perfectly OK with that. So life is good in that sense. On the other hand my ex is being terrible and bitter, says that if we cant be together we can not be friends or work together as parents in any respect says mean things about me to people, says that I "NEED to give him another F'n chance", uses the kids for information, interrogates them about me and my boyfriend instead of spending quality time with them. Has threated to call CPS on me say lies and have the children taken away from me and claims that he will fight me for custody of the children "just because he says so". So with all that being said, why do I sometimes find myself feeling sad, maybe depressed and missing...something, sometimes I feel that I just feel guilty that I've taken the kids father out of their life. Please give your kind advice.