awe your not stupid,,,, most training problems are silly things that you dont see your self, the other thing I would say is,,,,,you dont really have a sit problem from what I read,,,, but you have a heal problem, if your sit is first time every time, then I would work on heal,,,,but only treat when the dog is in close and straight,

I often get people at the training club who say my dog wont walk at heal,,,,always dragging me along,,,and so on, so I tell them to let the dog of lead to get used to the floor before I start training,,,,when the dog has had a good sniff, and wanders back over I ask them to tell the dog to go to heal, before they put the lead on,,,,,they always say,,,,he does not know how to do that,,,,my response is ,,,,if he does not know where heal is how can he walk there,,,,for all he knows heal could be up round the corner,,,or beside the dog he is trying to drag you too,,,,they always say,,,,I never thought of that,,,,lol,,,,so long as you have fun that is the main thing,,,,keep laughing, your dog will soon be fab xxx

i trained with treats with jrts and prts i stand and hold treat in left hand then hold it over the dogs head and say sit, i strted with the dog on lead and now they do it straight away. sorry if not much help. good luck,