How does one know that the field of research they are in is the best fit for them if they are average in it?
"I love computational materials science." End of discussion. 

If you can do what you love you can have a marvelous life. If you can't do what you love you will have a harder time. 

But, with time, hard work, and experience you will likely improve. There will always be people better than you at whatever you do. Don't worry about it. Don't envy them. But try to form collaborative relationships that might, in time, let you work with some of the superstars. 

As to your colleagues who seem to have an easier time: they may feel quite differently.