When your end goal seems unachievable, the way to get there is to set very easy goals, so you cannot fail as they are unfailable!  It could be to do x number of reps of door training a day, or x training sessions a week.  Maybe it's about getting out of the front door with your dog relaxed, or even go to the toilet without that little hairy visitor sat on your feet!  Here I talk about making habits and setting really easy goals.