Let me start by saying congratulations to you because you have a beautiful healthy daughter; try to wake up every day focused on that rather than the loss. She needs to see a happy smiling mama, one that is happy down in her heart and not one that is shining it on while she is angry and sad inside. This is the first place you start with tucking your son away, by remembering your daughter is the priority. Do remember your son every day and yes, your grieving is normal. Google the five stages of grief, you are currently pissed and again, that is normal. You just need to make sure that you don't let this loss color your entire attitude about life, etc., make you primarily sad, bitter, etc. forever because you are really doing him a disservice.
Couple of years ago, expert on grieving the loss of children was on Oprah I think. The program dealt with children lost to violent death but I think it applies regardless. If you only remember the death, that diminishes the life. In your case, by focusing on not being there to see the milestones, etc., your memories are all negative instead of seeing him as a soul that just wasn't here for very long.
No matter what we go through, we start with our minds and trying to get the right intellectual perspective, and then it is finally something that is accepted in our hearts. I think anyone that loses a child will, like you said, never be able to forget, etc. I think you try to move from totally devastated all the time to an intermittent bitter sweet. Your daughter will have milestones and at those, you will probably imagine what your son would have looked like, that will be normal and even healthy, as well as heart-wrenching. One of the things you do then is recognize that you have raised your daughter well, happy, healthy, that is how you honor your son.
I have dealt with loss, not that of a child but my father and grandmother, cousins, watched two of my cousins bury their sons, and I nearly died myself a year and a half ago. The prospect of leaving my children without a mother was horrifying and I honestly cannot imagine losing a child. While everyone experiences grief, allowing yourself to focus on the things that should have been will make the process take longer. I am not telling you not to do it, just try to monitor how much it dominates your thoughts. One reason why is your daughter. Are you really enjoying your daughter if you are thinking I should be playing with two babies? She won't know the difference but you will. These are moments you cannot get back and you may wind up kicking yourself later on.
And maybe here's how you start to get your head in a better place. It is not uncommon for one twin to be undernourished and either born terribly under the weight of the other or as in your case, stillborn or dying not long after they are born. I honestly don't remember if I came by this knowledge through a fictional program or something like 20/20 but I thought to myself at the time, the one gave it's life for the other; no greater love.
Another thing, God answers our prayers, just not always the what we seek. You prayed and prayed for years I assume for a child and you were blessed with that child, unfortunately a mixed blessing but this leaves you with another question. Why? That is inevitable, but start looking for positive answers. You said in your post that you were supposed to raise a girl and a boy, why don't you adopt a boy? Maybe this happened so that you could be a wonderful mother to a little boy whose life might not be so great without you. Maybe you are meant to foster children, that you have one child of your own but with the love and resources you have, you could help so many children.
With the support groups, try area hospitals and don't focus just on people with your particular situation. Families that have lost children are essentially dealing with the same thing and you may find the help and solace you want there.
I know it's hard and I know it is easy for other people to try to offer help but maybe the bottom line is that instead of looking at this situation as less than what you thought you were going to get, you should feel just the huge blessing that your daughter is. Difficult pregnancy, how much harder would this be if you had lost both babies? Some women are never so blessed so holding to that thought too might be helpful.
Maybe too pray to God to help you accept the loss gracefully? You know, grown-ups are not that different than children. Anyone responding to you is at a bit of a loss because if it were a friend we knew, we would tailor the suggestions to that person and the personality, the things that make sense to that individual. So like you would explain something to a child in a way that that particular child will understand and find meaning, you need to look for the way that you can look on this with some sense of the perhaps not positive but peace. That boy is in you, he is in your heart, and he is sitting at the right hand of God watching his beautiful sister grow up and he doesn't want his mommy to be sad.