I just wanted to add a couple of thoughts... I do not have children and have been with my husband for 9 years. Four years ago, my husband said he did not want, and never would want children, so I understand some of the thoughts going through your head. After many discussions, and some heated rows, it became clear that he was not going to change his mind (we HAD discussed things soon after we met, and husband said he would like kids one day). So, like you, I also felt 'cheated' out of our original 'deal'.
I then had to decide which I wanted more: the chance of becoming a mother or marriage to a man I love. I chose my marriage and have never regretted that decision. It took me a while, but I love my life and am very happy.
I am not saying that things will work out so well for you, as it seems that you are struggling to move forward. But I do feel very strongly that no one should be tricked into parenthood. It is not fair on anybody and I believe it is unforgiveable.
A couple of our friends were in a very similar situation, but the husband wanted a second child, and the wife didn't want it at all. Anyway after lots of discussions/ rows/ nagging she eventually gave in and her little boy now has a baby brother. She is a lovely perrson and tries to hide things, but i think it shows in subtle ways that she just didnt want a second child, and it is clear that she does not really enjoy motherhood anymore. What an awful price to pay, and that child will probably pick up on some of these signs when he is older.
Incidentally, my mother (due to mental health problems) made it clear every day of our lives that we weren't wanted and I know the longterm pain that this sort of rejection as a child can have on somebody. i still struggle with confidence and relationship issues and I am 35.
Funnily enough there is a big age gap between me and my siblings (I am the oldest) and there is only two years age gap between my brother and sister. They both get along well with me, but spent much time as children squabbling and not getting along, and even now only really tolerate each other. There is no strong bond between them whatsoever.
You have no guarantees and there are several assumptions in what you think life will be like if you have a second child. You could just end up a single parent, with your child resenting you, and even if you have a second child you may find that they just don't get along. Maybe in years to come your child will wish that her parents were happily married and she was an only child!