the open relationship will always fail if you two are not mutually stoked on doing it. that probably seems obvious, but i'll explain a little.
like, it has to excite you two. that you are together and encouraging each other to explore sex / romance with others.
more than excitement, this has got to be something that turns you two on. makes the attraction between each other stronger in a way. like it deepens the bond you have, adds some kinky magic to it. and it has to be something you are super stoked to talk about. and talk about very often! intertwining the sex-life-aspect with the communication-aspect. its like being hyped up bandmates when things are working best. communicating and interacting without missing a beat. its pretty tight and worth the communication that goes into it, when its working.
I don't have extensive experience with open relationships, but this was definitely the best thing about it. The communication and openness was really refreshing at the time, and it felt like we were close in a different way than before. We could joke with each other about the people we were seeing and trade advice and stories. It made us feel even more like best friends.
But - that relationship ended because she wanted to be with someone else. That was over two years ago, and they just got engaged. I feel like most people would see this as a sign to never be in an open relationship, but I think it worked out for the best. We're both happier now, and with people who fit our needs a little better. There were parts of that open relationship I regret, but most of those were little mistakes that I made along the way. I'm glad that I got the experience to do it.