I take it you don't have any children and if you do.....wow.
Most of us here are parents, and I think are pretty well in agreement that you are doing your child a grave disservice (and have done from the start) by allowing her to believe he is her father. The wow should be directed at yourself, to be honest.
She is too young for me to do that. I'll tell her when she's old enough to understand what that even means......
Sadly, you have set her up for this emotional trauma by not being honest with her from the get-go. It did not have to be this way, had you been honest from the start. My neighbors have three children, of which the youngest two are adopted. The middle one at ~a year, the younger at 9mos. They are now 3 and 4, and the oldest is 5 1/2 (S/N). All three know the truth of their parentage - and have from the beginning. But none of them feel "less than". There is no shame in them for how they became a family.
At this point, you really need to give serious thought to finding professional help for your child. She needs to know the truth, and you are not equipped to help her at this point. But, a professional can get you both there. Please - for her sake - seek that help. The rest of your issues? Are frankly minor to this one.