Am I Smart Enough?

An issue I've had for a long time has been doubting if am I smart enough for what I want to do in my life. When I think about it by myself I believe that I am smart enough because I've never really had any trouble with my school work and if I did I could figure it out on my own. Being sort of a loner has always been good for me academic wise because I have lots of time to do homework and I have to figure problems out on my own because I have no one to help me. I've gotten to the point in my school work where not even my older brother can help me out so I'm completely on my own.

However, I'm constantly doubting whether or not I can do what I want to do when I discuss it with other people, especially relatives. Because you know how relatives always ask you the same boring questions like, "How's school?" or "What do you want to do after school?" I tell them that I'm going for a science degree and they are always a bit shocked (which to be honest is hurtful) but then they go on to say, "Are you sure?","You know it's quite hard." I always answer, "Oh it's fine I'm good at math and science." But then they say, "But you have to be really good." And I reply, "I am."

After the conversation is over I'll begin to question myself on whether or not I can handle it. Or if I am good at math or if I really am smart enough. It's those thoughts that absolutely ruin me. Like I said I've never had a problem in school. I'm a good student and I grasp subjects quite easily. However, when people tell me it's hard and that I may not be able to handle it, I start to believe what they say. Obviously, this is stupid because they haven't done science degrees and they most definitely don't have the same mind as me. I truly believe I can do what I aspire to but it's hard to not doubt yourself when people are constantly doubting you.