I do understand you so well, justAlover, because I had been in quite a similar situation for years. And even after my boyfriend's wife opened up the relationship and explicitly included me into their family I still had a lot of difficulties not to see their relationship as more important than mine for a long time. But what helped me a lot was that it was his wife's decision that I could be with him and I think that's what you'd need it to be as well: his wife's approval coming from herself, so that it doesn't feel to you like she's been talked into it. Now, of course, I cannot guarantee you that it works out in the same way as it did for me (specifically because I never intended it to) but I think the most important (and at the moment the only) thing you can do if you do not want to break up is being patient and honest to both of them. And that also means that you must tell him about how you feel. If you are so important to him (and I deduced that you are), he will take that as serious as if his wife was talking to him about her feelings. Of course, there're times when the wife is more important, but there're also times when friends need more attention and dealing with that is his job, not primarily yours, though you need to take into account that he might be occupied dealing with other matters when you want him to be there for you (but so does his wife or anybody else), but remember that there will be the time when your concerns are most important. It is hard to handle two relationships, but if that is his decision he has to deal with it the same way you do.
And concerning his wife I don't know what exacly is your relationship to her. Are you just acquaintances, like you meet each other now and then just because you're a friend of her husband? Or are you friends? To tell my story: My boyfriend never kept me from his wife and so she did not only know I was really important to him, but also did we have a chance to become really close friends. Sooner or later we would find ourselves joking around making him pretend to be annoyed (though he'd in fact be pleased that we get along with each other so well). It has always been a beautiful and special friendship to me, because it also allowed me to be close to the man I love and I always felt welcomed. And so did I grow on her and because I was a good friend of her she wanted me to be happy and of course she knew how much her husband and I loved each other (but we were never ever cheating on her in any way, I am not able to do that to her), so this extraordinarily remarkable woman decided to make us both happy. But that took a long time (about five years) - I've had other boyfriends in the meantime, but I never gave up on him, though I never imagined things could work out like that. My future plans consisted of us being very close friends and that thought made me happy, though I also had my troubles learning this "compersion" as it is called among polyamorous people. I never expected it to work out even more beautifully, but it did.
There's always hope. Just don't give up on him... and her! Give them all the love you can - without giving up on yourself, of course - and be happy about every piece of love that is returned. But don't become to set on this to work out like it did for me for instance. Instead be open for anything. That's all the advice I dare to give.
P.S. I think it's actually great you've come that far to be able to love someone though he can't return that love in the same way!
P.P.S. As you probably realised English is not my native language and I tend to have troubles expressing things like that in English. I hope, despite of that, you understand what I try to tell you. If you're not sure, go ahead and ask me