Your arm is unlikely to be totally useless. Much of the advice you are given is calibrated to fit the worst possible outcomes, which sort of makes sense , I guess. Hope for the best and plan for the worst. There's nothing worse than being told you will be fine and then discovering you are unable to manage--this whole business comes with inbuilt disappointment without giving anyone unrealistic expectations
Surgical outcomes vary so radically that it is not possible to predict how you will feel afterwards. Responses to anaesthetic, seroma and fluid retention, even silly things like dissolvable stitches not behaving can all slow down recovery but are totally unpredictable. It's reasonable to plan for discomfort but don't get too stressed. Lumpectomies are generally not too bad and sentinel node procedures have varying results depending on where the node is and what they find.
Good luck and set yourself up so you won't be doing anything too vigorous for a little while. You may be surprised at how quickly you recover.