You are right Christina when you say
excruciating exercise in patience.
But in the end you did good.
If you have a layer mask it is like an adjustment layer and you paint in the changes with black and if you go to fasr you correct that with white paint and try again. I did it the destructive way where you cannot correct mistakes except immediately with the undo button. But I don't know what programme you are using and I use Paint Shop Pro so it is likely though similar there are serious differences.