How can I stop letting my fiance's ex-gf make me so mad?

I have only seen this "woman" twice and the thought if her just pisses me off. First of all, my fiance & I have been together about 2 years total & have lived together for almost a year. Our children live with us full-time (which I prefer). That being said our dtr is 7 but is mine from my first marriage. Our son is 4 but is his from his ex-gf. So, my dtrs father is not in the picture (his choice...he lives at least 20 hours away), and sons mother is not in the picture (except to call and cry to my fiance about how much she misses her son & wants to see him so bad....she lives about 24 hours away). Mind you, we do not keep the children from these people, they just choose not to make their children a priority in their lives. We do & we love each others children like our own. I know I should probably be grateful that I don't have to deal with son's mom but she makes me so mad that she can just get up & leave her child & not make an effort to support him in anyway at all. I went through this with my dtrs father & it just breaks my heart to see another child go through the same thing & it takes a lot out of me emotionally to go through this again. I could almost understand a father doing this to their child...but a mother doing this just blows me away! I could never! Although, no parent should be able to...mom or dad....its not right! Has anyone else raised another woman's child....similar to this situation? If so, how do you or did you release this anger towards them? I still have issues towards my dtrs dad but it has gotten better with time. Thanks for any suggestions!