I’m sorry this happened to you! Yes, I think workouts need not be painful. It’s best to ease into it and ramp up according to what’s comfortable until you hit your goal, rather than trying to make drastic changes real fast. It would be good if more people were aware of this. I’m sorry you had to learn the hard way!
You probably don’t actually have NPD (although there’s a small chance you do). The majority of diagnoses of personality disorders and mental health disorders are not realistic, but just labels given by a deeply disturbed medical system that looks at the tremendous pain caused by bad social and economic conditions and blames the person as being “ill” instead of identifying problems in their environment. The world we live in is rampant with narcissism – if you happen to personally reflect that sometimes, that makes you more normal than abnormal.
I’m guessing your actual problem is that you’re stressed, for reasons such as: an unhealthy upbringing, a shitty job, poor educational resources, lack of money, low-quality healthcare, lack of supportive relationships, uncomfortable living arrangements, etc. – take your pick and think about what’s relevant to you. It is a crazy fucking world we live in, and just in order to get by, people often adopt narcissistic and selfish ways of thinking and acting even if that is not how they feel in their heart of hearts.
I don’t know what age you are either (and I’m not asking you to tell me), but the younger you are, the more likely you are to get caught up in misleading notions that you’re crazy because people are giving you shit information. Just work on being healthy and improving your life and treating others with respect. You do not need a label, nor do you need to worry about what people will think of you because of said label. Fuck labels. You’re a human being. Doctors may be able to give useful information about illnesses with evident physical markers such as diabetes or multiple sclerosis, but in most cases, they have no business talking about what’s in your head.
I was advised by a group therapist, when having “cold feet” about commitment in a relationship, to just think about how bad the alternative might be. She had a friend who had reservations about getting married, and then thought not getting married would be stressful too, so she went ahead with the marriage. It was terrible advice, especially from a therapist providing guidance for some very troubled people. The only good reason to commit to a relationship is because you really want it, not because you’re afraid of what will happen otherwise!