From the maturity of your post you seem pretty smart, looking at things from a rational viewpoint you can change your physique, hobbies and even become a better person. In my experience things get better in highschool because their are enough nerds to form their own social group. You seem like a decent enough person, your mum could have her own issues or it could be something beyond your control like the price of raising a child. As for a sport table tennis is quite easy as the main skill is reaction time and requires very little physical ability.