this has been bugging me lately....

Somethings been bugging me lately. Well, its been happening all my life, i cant focus well. Mostly on school things, but also on street smarts too. My sister has known the streets since she was 10, and finishes her homework accurately and quickly. I dont know the streets that well and im 15, and i dont understand school as well as i would like too. Why is it that other people can focus and do things better, and i can barely do a fracticion of what my 10 year old sister could do. (Well, shes 17 now, but she knew way more than me street wise when she was 10) Can someone help me please? Im feeling really alone and lost.
Edit: my mom is comparing me to my sister as well, and so are many other people. Some people are starting to think something is wrong with me..