First of all congratulations on your baby!! It is true that often people has miscarriages because of bicornate uterus with a septum. You are very lucky to have had your child without knowing about it or having it fixed but don't worry you can have it operated on and have more children
I had a bicornate uterus with a septum and had laparoscopy in Sept to have it fixed. In Dec I had my ET and got pregnant with triplets. I have had a great pregnancy with no cramping or spotting and my cervix is still long and closed. My uterus is holding up great because I had the operation to fix it. The operation was done by my RE. I was put out and basically my doctor went in through my uterus and inserted an instrument that cut away the septum. The recovering was quick and not painful. I bleed a little but only for a day or two.
I am sorry that your OB scared you about this. You do need to find a doctor that has experience with this type of surgery. My RE told me that it is a simply procedure for doctors who know what they are doing but it can create additional problems if it is not done right and they cut through the uterus wall.
I wish you all the best and again congratulations on your baby. Don't be sad...a good doctor can do wonders!!