I don't really fit into the standard pattern, as I first suggested opening up the relationship as an alternative to just breaking up. I had come to the realization that I wanted something from my partner (G) that he could not give - notably intellectual conversations and well thought out opinions. He is the kindest, loveliest man you can imagine, but intellect is not his strong point. We had struggled with that point for many years, but no amount of relationship talks made it go away. At the time I said: "Either we open up the relationship so I can get my intellectual stimulation somewhere else or we break up" He answered that he didn't think that either of us had the stomach to open the relationship, so it would be better to just break up. I disagreed, but didn't say anything at the time.
After that I spent a lot of time with T, my best friend, who had also listened to all my problems with G over the years. G and I were still living together and I needed to gain some distance and perspective. At that stage I knew though that I didn't think monogamy would ever work for me again, as I was expecting everything from one person, which is simply unrealistic. There would NEVER be a person to fulfill all my needs. After a while T and I started sleeping with each other, but I made it clear that I didn't want a serious relationship at the moment and that even if there would be one, it would not be mono. T accepted that and said that from what he saw in his circle of friends, poly worked better anyway and he was happy in a poly set up. I was his first everything so he didn't really have an previous experience he could take a preference from.
While I was over at T's, G came to visit us with our little dog and we ended up having more fun than I had had in ages. G and I actually had a few honest conversations and some genuine fun, which hadn't happened in ages. T made a real effort to get on well with G, who he so far had been mainly friends with as my boyfriend. Thus the idea came to me that may be the opening up part wasn't so ludicrous after all. When I got back home, I asked G if he'd be willing to give it another try, but this time in a poly relationship. He had a long hard think about it for several weeks and then essentially said that he wasn't willing to say he couldn't do it until he had tried it - however he would only be comfortable with T as my other boyfriend and he didn't want to be in a relationship with more than 3, thus essentially limiting T to being mono and me to never exploring beyond a Vee.
It took T and me a lot of thinking and discussion before we agreed to it, at which stage we started a "test run". At the beginning G kept saying things like "if this works" or "if we are still together by then" which really hurt me, as I was trying hard to rebuild our original relationship while also being Ts first girlfriend and splitting my time evenly. After about a month of him doing that, I told him that I felt he was being really unfair as he expected me and T to commit ourselves completely emotionally, but wasn't willing to do so himself. That lead to some more thinking and him agreeing to not do it anymore.
We have been doing fairly well ever since. G is still a bit uncomfortable with two people being romantic in the presence of the third, but he says that he will really just need time. Actually with most things he was uncomfortable with - time solved the problem. We are looking to move in together in August. Gs main worry was to become a third wheel, so I have put a lot of effort into making sure he doesn't and I split myself more or less even.
G has gained both a more relaxed me and not getting shouted at if he doesn't have an opinion. I still ask his opinion, but if he doesn't have one I just move on. He also enjoys listening to other people chatter on high brow topics more than saying anything himself, so he quite enjoys listening to T and me. On top of that, he has gained a close male friend, which I think actually means quite a lot to him, as he can be very socially awkward and has a hard time making friends. He has two female friends at work, but that is more or less it. He really enjoys his "guy" time with T and having someone who is enthusiastic about similar things. I also have a fairly precarious health and regularly need to be looked after, so having someone to share that with is much more relaxing for him.
T has gained a girlfriend and the fact I don't get upset with him if he is being antisocial or doesn't want to do certain things. I just do them with G instead. He has also gained a dog, although I am not too sure how happy he is with that part :P He's lost the ability to search for someone else - potentially a boyfriend (he's bi), but says that that doesn't really bother him.
TL;DR: I suggested opening the relationship to G before breaking up. He said no and we broke up. I got together with T who knew I'd want to be poly. G and I decided to give it another try after lots of thinking. He put down very strict ground rules. He was initially unwilling to commit emotionally, but changed that after we had a chat. We've been happy as a closed, non hierarchical MFM Vee since.
G has gained a close friend and a much happier girlfriend I feel, as well as someone to help him care for ill girlfriend for girlfriend is ill a lot. He also now doesn't feel like he has to fulfill all my needs anymore which is much more relaxing for him.