I can really relate to what you are going through. I was 22 with my first son, and 33 when son #2 was born. It was not our idea of ideal, but my husband and I are both very thankful for the children we have. It is challenging to raise them with the age gap, but there are advantages to. You have the chance to devote more time to the 2nd one because the older child is more self reliant. Your daughter may get jealous or you may have the opportunity to show her what being a mother is all about. Your daughter is old enough, to appreciate the lessons and understand more, than if she was much younger.
You'll be a better mother now that you are older and more mature. But it is a BIG change to go from one child of a 12 year old to a baby in the house. You really have to want to change the dynamics of your family. I worried about all that but it all works itself out and now I cannot imagine our life without our youngest son. He has alot of spark that keeps me on my toes but I'm more equipped to handle that at this older age than in my 20's. Also, I recognize now when it is wise to ask friends for advise or find books on parenting topics.
Both of our sons, are a blessing. The age gap of 11 years has made life more interesting. They are 11 and 23 now and still share a bedroom. It is great to see them go out together to go play basketball together. Our oldest son also is able give his youngest brother with homework.
I wish you the best. Make sure you consider all sides before you make a decision. In the end, it the choice of you and your husband.
We've been married 26 years.