I'm sorry you felt there was no other way than to end your relationship, that's really sad. I suppose everybody has their limit and you felt yours had been reached, with good reason by the sounds of it.
Most relationships grow to accommodate new relationships - whether it's your ex DP adapting his approach to his ex and his DD to accommodate your needs, or a couple growing their relationship to bring a new child into the world. On both counts it takes a lot of hard work and sometimes fails.
Not sure if it's any help or not but i'm pleased i gave my relationship with my DH a few goes, otherwise we would not now have our wonderful 18mo DS. We are (mostly) extremely happy. The downside to this is that we are both now struggling to come to terms with the fact that his eldest DD has chosen not to have a relationship with him (not because of DS, but because she has been systematically alienated over many years by her mother who effectively allowed her to choose whether or not she saw my DH).
My attitude was always that i loved him and knew, sort of, what i was getting into - i.e. i knew he had two DDs. I also knew his ex could be, ahem, difficult. But i made the choice that i had never loved anyone like DH and probably never would again, so we worked through things. That said, he has always been able to listen to me and adapted to my 'needs' - often requiring compromises on both sides. The first few years were difficult and actually not really because of his ex and DDs. He suffered from depression because he missed his DDs which took a tremendous toll on our relationship. We separated and got back together several times - a real case of 'can't live, can't live without'. In the end i gave him an ultimatum: shape up or i'm shipping out. For Good. That was it and we've never looked back. Today, we have been together for ten years.
Nobody can really advise you what to do unfortunately because it's so, so subjective. I wanted to give you a different conclusion in the hope it might also help. Fwiw, i'd do it all again tomorrow and we've had many, many challenges along the way. The way i see it, if you got to the end of your life and you were offered the chance to do it again, wouldn't you take the opportunity, despite remembering the many bad times? In relationships i don't think opting out when the going gets bad is necessarily the right thing to do - though there are clearly exceptional circumstances. It depends where you draw the line personally, and i think you have probably drawn it?