Your father is probably just worried about you and, to be honest, lots of medic parents are now actively dissuading their children from entering the profession. It doesn't sound as if you need to make a decision in the immediate future if you don't have the grades to be competitive at undergraduate entry and so will complete a BSc first anyway. If you are still keen then I would continue doing the various things that are necessary for a medical school application (volunteering, shadowing, etc) while actively exploring other careers as well. To be honest, I would be very impressed to read a personal statement from someone that had explored various careers (academic research, industry science, banking, teaching, or whatever else takes your fancy...) and could then explain their reasons for settling on medicine. You might find that you become more enthusiastic about something else but, if not, medicine probably is right for you after all.
The one stress that is unavoidable as a doctor is exams but you will get a good feel during your BSc how you react to them. Exams at medical school will be a bit different (emphasising breadth at the expense of depth) but are unlikely to be harder. The first couple of years working as a doctor can also be stressful but they are soon over (most people enjoy them anyway) and you can soon tailor your career towards something less stressful if necessary. However, I suspect that, like everyone else, you will just find a way to manage the "stress" and would get on fine as long as you're enthusiastic and willing to work hard.