Unfortunately I do not have time to read every single post on here, but I did peruse most. I am going to totally walk into unknown territory here (being not poly really at all, merely interested) and try to put in my two cents.
It sounds to me that, Ceoli, you are wanting a relationship in which you and your partner(s) are there from the beginning. I can understand the underlying feeling of being an outsider still when dating a couple (of course - they have this whole history together you were not involved with!).
It seems to me that (and I am going to COMPLETELY put poly aside for right now) that you desire a life partner. Whether or not there is one or two of them, you want someone to have a beginning with, a middle with, and a foreseeable future with. That is what a lot of singles desire, poly or not.
Reading through these boards over the last few months, I have totally noticed that there is a DEFINITE prevelance of couple-based poly folks. I think I even mentioned in a post of mine once, what the heck do you do if you're NOT already in a couple??
I guess I am getting an answer to that question by reading your thread here. I wish I had some magic words of wisdom, but alas, I am in a rock-and-a-hard-place situation myself being a single mother. Men don't mind dating single moms, but they won't commit to them.
Being single is pretty rough. On the one hand it's nice to have freedom, but it's also nice to have a partnership. Here is a site I want to refer you to. Maybe it's too cheesmo or doesn't have to do with your situation, but here it is anyway if you want to look at it: