I think you are experiencing feelings of cold feet. I think it is normal to have these feelings after all you have been through and the loss of your genetics is real and it tugs at your heart. I think that grief can strike us at odd times and it is like an old boyfriend that we still long for. Could of , would of, should of... but that time is over and now you have your baby that you worked so hard for.
I too have had feelings of grief, cold feet and regret all wk. I almost canceled my Dr Gill appt and then once I picked out my donor. I felt sick like maybe I should not be doing this. I got this same sick feeling just prior to our wedding day. Cold Feet. Every major decision makes me feel this way as so much is riding on it and we can not undo it once it is done. So you are normal and it is better to work through your feelings now than later.
Remember you have the baby inside you that you are suppose to have. This is all so hard and foreign to us as we grew up thinking we would have our families one way and to switch from 40+ yrs of thinking is difficult and complex.
Allow yourself to feel so that you can heal.