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. I was an experimentalist and involved in some Computational work during my masters degree. I did not not enjoy the experimental part. I did not enjoy the lab politics, the dependency on others. Also, I did not enjoy the significance of the work I was doing. Most of the time, it was just reporting the experimental data without trying hard to find a theoretical reasoning. 

On the other hand, I was fascinated by computational research. The only limitation was I myself. Implementation of ideas, thinking about the fundamental physics that's occuring, and getting to theoretically predict the results was exciting. Thus, I was extremely determined to do a PhD in computational aspect of materials science for this particular reason. 

Fast forward 4 years, I am done with my PhD. And I have published some papers on my research area. However, I did not do anything substantial during my PhD. I did not get any awards, nor my research got any media coverage, applying for grants with my advisor was always a nightmare. Contrarily, my colleagues who work on experimental aspect of the research, find it much easier to publish, gets multiple awards, easily gets grants approved, have media coverage for every "groundbreaking" stuff they develop.

Now that I have completed my PhD, I sort of envy the experimentalists and regret not being one. I feel that being average in computational materials science is disasterous as compared to being average in experimental research. I feel that I made a wrong choice 4 years ago when I decided to apply for a computational PhD than an experimental one.

My question is: How does one know that the field of research they are in is the best fit for them ? And how to be content with one own's research field and not feel resentment that probably I am not a good fit for this field?
"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.