Irrational Fears

Why is it that past relationship failures continue to haunt? I know that Marty loves me. I know that with all my heart. And I know that there's no romantic tie with him and his ex. At least on his part. But they're still close and that's bothersome. I guess it's hard for me to comprehend that. Though I've always had pretty decent, clean breakups where I've remained friends with my ex-boyfriends etc, I've not had it where I would still spend time doing things with them and remain really close. There's enough emotional upset to keep that at a distance. And perhaps Marty is just too nice of a guy. I can understand remaining on friendly terms for the sake of the kids and just to keep things nice, but for me, I would have to draw the line somewhere. I know she's been using his kindness and he's allowed her to do so. I know that lately he's trying to break that by not going over there so much. But they still do a lot together. He took her the doctor. They went to a mutual friend's house for dinner and let the girls play with other kids. He goes over there for dinner. They play pool on a team every Wednesday. And I know that Marty is so nice, he has a hard time saying no. He has the fear that his ex will try to keep the girls away from him if he starts saying "no" too much. I don't know his ex, but from the sound of it, she might do something like that just to be spiteful. And that type of women really pisses me off. I've been spiteful and vindictive, but not when it comes to my child. It just seems that in a lot of ways, his ex still has Marty tied around her little finger and I have a hard time sharing like that.
Marty said tonight that I'm treating him like he's like Richard. I don't mean to. I do trust him, but I'm still uneasy about the tie with his ex. I can't help it. And yes, I think that I'm still affected by my past. I trust Marty probably more than any other man. I don't trust his ex. I don't like the situation. And being 1500 miles away makes it all the more difficult. And it's completely irrational and I know that too. But it doesn't make it any easier.
And there's nothing that can be done. I can't make demands on him because of my silly fears. And I don't even know what that fear really is. Do I think Marty is going to go back to his ex? Not at all. Then why does it still bother me? So I guess I just have to "get over it". Perhaps when I visit next month and I meet the ex I can put some of the fears behind me. I just hope it doesn't raise any more.
Can anyone explain to me why women have irrational fears?