Hello and welcome,
I am sorry that you are hurting. I know how arduous it is emotional wise to be in this kind of situation; my start was similar but a lot more positive in the outcome.
However, there was something missing in your post. I don't know why and maybe I overread it kind of, but what about your pure feelings towards your boyfriend? All I got was your fear of losing your 'perfect and ideal future' not your fear of losing him. You are afraid to start over, to be alone. But I think that your motivation for being afraid is somehow not that positive towards your partner. You aren't saying that you need him especially, you say that you need to have someone to fit your picture perfect family image. I would feel hurt and insecure in this kind of situation if I were your partner, because I wouldn't feel valued for who I am, just for what I can provide.
Than the thing your boyfriend is having trouble with. It's really common to be hurt because one feels like 'I haven't been enough'. My husband went through this phase as well. It's the realization that no one can be 'the world and everything' for another. I went through this the other way round (being the one and only for two men at the same time is kind of stressing when you think about the no one is able to be all to his partner statement ...) and I am still waiting for them to realize that they need more in some moments. What helped in our situation was acknowledging that no one can love another for more than he/she simply is. If you love your partner, this love will not fade just because there has been another with some different traits or even some similar ones that awoke some feelings of love. Those won't change the feelings that have been there already (given the case that you are poly, of course
). My husband didn't just believe me and said "Well ok, if you say so." I kind of 'proved' it to him by time. He didn't feel loved less and that was when he was able to make his peace with the way I was. Some things aren't understandable by logic alone, we need some experience to learn as well.
That's why I would recommend to take a deep breath and keep in mind that things need time. All you can ask of your partner right now is keeping the line of communication open and be willing to talk about the whole mess. You can't expect him to be suddenly happy by your revelation, saying 'Let's get things started, no problem at all on my side.' Because this needs some time to be processed. If he indeed feels that he never will be able to accept any of that was has proven to be your nature, you indeed have a hard choice in front of you. If you two are unable to work it out, you need to start on building a relationship, you seem to have neglected so far: with yourself. As long as you fear to be alone, you will be a clingy partner who isn't able to provide happiness for herself, solely living from the energy she gets from her spouse.
I can say that I would be horrified to lose one of my partners; but not because I am afraid to be alone - because I would feel like something irreplaceable is suddenly missing. And there is a huge difference between those two.
Facts: 30, female, bi, v-type relationship with Sward (husband, straight, mono) and Lin (boyfriend, straight, mono), poly-fi and co-primary.