I have the same concerns as you, but i am trying to tell myself that i didnt know what i was doing last time, i have learned the better/gentler way to do things now, so hopefully this time around i will know what to do... but maybe not.

I think that it is normal for everyone to have these concerns, just the same as in the lead up to the first one, reality sort of hits and you thing OMG WHAT am i doing? HOW will i cope?
i think that it going to be adjustment.

And i dont think that you will stuff up P's life, you are going to give her the sibling that she will really want and need at some point (MJ is desperate for a little playmate now and DSS would have loved to have someone close to his age) Of course there will be an adjustment time, but there will be so many more positives for her than negatives in the long run.

Tell your Mum to keep her comments to herself, its hard enough these thoughs entering your head at times, but being told them is just totally uncalled for