The problem is not that your child’s IQ is 81 but that she is being raised by parents whose IQ is much higher. Biological parents have some kind of special gift for their own children, no matter what their IQ.
This is not a calamity. I think you are suffering right now from a sudden encounter with the facts, but you can compensate for this and have a wonderful, competent child. Probably you have to simplify your initial impulses in some way.
The challenge is probably going to be a great gift to you and your child. One person’s opinion, anyway.
I agree with firebrand. I don’t think anyone here is going to say there are easy solutions. The good thing is that you love your daughter and want to do what is best for her.