I wouldn't automatically put yourself into the average intelligence group. The fact that you are able to do Physics at a C means you are already way above average.
I always use to think that Intelligence was a binary black and white thing. Constantly struggling at school, understanding things in the moment but never able to recall and apply the knowledge effectively to get good grades. People would always think I was intelligent by the way I talked and thought, but looking at my grades or ability to manage daily/mundane tasks and you could see something very wrong.
I read book after book, learned about the brain, learning, memory and plethora of things in between. After developing negative feedback loops with everything, I generally accepted that I would be a failure at everything.
One day I came across a book that just explained what was fundamentally wrong. Long story short my brain couldn't focus and never did, it never developed structures necessary for success in anything let alone math, physics, computer science, music. I found a very very special treatment that can change the network in your brain responsible for focus and enhanced it.
Then all those years spending learning about improving abilities was all accessible, like someone flipped a switch and everything I read/experienced was there. Now I spend my days excelling at everything, there is not a single thing that I am not above average at and in some things far far above average. I live the Limitless movie(although its taken a year for it to be really evident) everyday of my life.
My brain has changed so fundamentally, I can't even say I am the same human being anymore. Now I spend my days trying to become a "genius" I always dreamed about and starting to understand that intelligence isn't one thing in particular its many things:
1. Focus
2. Sensory Awareness
3. Reaction time
4. Analytical Ability
5. Working and Long-term Memory
6. Creativity
7. Emotional Intelligence
8. Executive Function/Task Management
There are probably more, but those are the ones I focus my attention on. You have to find ways to improve those things, it might take a year or it might take the rest of your life. I make it my life mission to identify geniuses in any domain and learn from them. Most of my learning has come from a Kung Fu and Dance master and he excels in all these areas.
Just remember that your abilities are heavily dependent on your intellectual/emotional development. If your parents have serious limitations in those areas you will probably have them to because in your core development years you were mirroring their neurons, but that doesn't mean it can't change. The hardest part is identifying the limitations.
The brain is infinitely complex, everything you have perceived/experienced is probably there, its just a matter of how you access it. You have a universe in your brain use it. I have really only scratched the surface, because even knowing all these things is not enough. You actually have to do physically/mentally stimulating activities to develop these abilities. Even then it is luck, but not impossible.
I am not trying to inflate my ego(because that's a weakness), I am not a genius or anything special in my mind. But it is a just the reality I worked tirelessly to create and I like to share that experience.
Edited For Brevity