I'm so sorry you are hurting. That really sucks. There isn't much you can do except look for someone that won't treat you as a secondary or at least the way this woman did. Someone that sees you as equal to whomever else they are with in life. of course if this was not meant to be then it wasn't meant to be, but as they said that it had nothing to do with you, then I suspect it was because you are a secondary.
In my opinion hierarchies suck and should not be sought out. They can be destructive and painful as they assume that secondaries are not worth as much love, are only there when the people involved want them to be, are disposable at the whim of the "couple" or other two involved and their life choices and evolve-ment in the relationship is not worthy of discussion or respect.
There are some great threads here that you could read that will give you some idea of what other have gone through and might give you a sense of belonging in terms of your emotional pain. Here are a few threads I found by doing a tag search for "secondary" and "secondaries"
secondaries bill of rights
The only time I have seen secondaries work is if the secondary has their own thing going on and doesn't want to be that involved. Usually its a partner that is in it for the sex or just because they want the company occasionally and they are free to pursue whatever avenue in life they see opening to them. If this is a known right from the beginning because everyone is communicating openly and honestly about what they are hoping to achieve in their life in terms of needs then it can work. It sounds like you didn't fit this and there wasn't an awareness on their behalf... of course it could of been that all of you didn't know or something changed. Communication is so important.
The other way I have seen it work is when the secondary likes to be involved with a family or couple and help them out and become involved with everything they do, knowing that they either don't want a family or primary of their own right now or ever and they are quite happy being "aunty" "uncle" to the kids and being the partner of one or both of the people involved. Very rare and very particular to certain people I think. Its not a common way of being and I would actually argue if they really are "secondaries" at that point.
*hugs* to you.... hopefully you find some solace in the fact that you are not alone and can move on.