Your last statement is absolutely correct. God has a plan for your life and you are refusing to accept it - as we all often do.
But let me put something in perspective for you. My best friend got pregnant 5 years ago, had to terminate the pregnancy at 21 wks. Then got pregnant through invitro 3 years later and miscarried right away (they paid for it by taking every penny out of their 401K) and then tried invitro again last year and miscarried again. Til this day she doesn't even have ONE child.
And you have an amazing son that was a gift from heaven. How blessed are you to just have him? And I know it's easier said then felt at times but we have to remind ourselves of these things and once we continue to tell ourselves over and over, the words that come out of our mouths and into our heads will become our feelings.
And lets talk about the perks of only having one:
*Less money to save for college, wedding, etc.. and more to spend on vacations and so on
*No having to loser pregnancy weight (this one is vein but nonetheless)
*More time for you and your husband
*More individualized attention for your son
*More "you" time
And we could think of many more.... But I think you have to keep going back to the fact that you are so blessed to have what you have and that some families are meant to be single child families. Not because of the reasons you stated, but just b/c that's the dynamic that is supposed to be played out. It's perhaps the way that he is SUPPOSED to be shaped - as a child w/o a sibling. Sure there will be some downfalls but I know people that LOVED being a single child.
And the more you press the pregnancy fact with your husband the less likely it will happen b/c of your stress and the more tention will come between you.
So release and let GOD....
And you never know... My neighbor has a 12 year old and they too tried to get pregnant for a few years after he was born and it never happened and all of a sudden - SURPRISE - 12 years later she got pregnant accidentally. Not the ideal space between the kids but God has a sense of humor too.
Just hang in there and don't beat yourself up for having these feelings. Just remind yourself of the things I stated.
It may also help to talk to a counselor.