Relationships are doomed to end, not least because lifetimes end. Personally, I'm inclined to view longevity as just one axis along which to measure a relationship. It's a quantitative axis, not a qualitative one. A good concert, a good crop or a good breakfast isn't any less good because it ended.
Multiple-partner and open relationships are less supported by the world in general and face more pressures, yet many outlast more traditional structures. (Mine have, but that may be as much a testament to the incredible, stubborn, loving patience of my partners than any other factor.)
I do have suggestions for starting or staying on a path toward better open relationships. The three-point summary:
That's the core of what I have to offer. It's not news or rocket science, and it's not very specific. I've also got a few more specific suggestions if you're in for a longer read. If you're in a rush, most of my responses at the above link boil down to the suggestions most people would give to anyone entering into or trying to maintain a relationship: "Listen, care, take responsibility for your actions, ask for what you want, bail if it's obviously failing and work like hell otherwise." That and "Don't be a shithead."
More gentle, less didactic responses to similar questions can be found all over the web and in many public libraries. My sole qualification on this front is that having been steadily in open relationships since about 1983 and having been surrounded by communities doing similar stuff for most of that time, I've had more opportunities to make (and observe) more mistakes in open and multi-partner relationships than many people. Please note that making lots of mistakes for a long time does not in most fields constitute expertise.
My favourite critique of polyamory is at: http://www.zazzle.ca/polyamory_is_wrong_tshirt-235838933475364492