How do you come to terms with the fact that you might never be among the best in your research community?

If every swimmer were to compete with the latest 20 year old Olympic gold medalist, they would soon give up. That's why we have different leagues. They provide more relevant frames of reference to compare different athletes. Thus you need to:

Shift the frame.

Adjusting a bad frame:

You are a PhD student who completed some regular college and an outsider to your field. But you are comparing yourself with your supervisor and with "the brightest kids" in the world and, what is more, with scientist who specialized in your field from the get-go. How can that not be disappointing?

What about: You are an up-and-coming young researcher with real-world experience and a background that is not the usual run-of-the-mill in the field you are working in. You have an amazing supervisor who trusts your work and you are at least as successful as the other PhD students in your cohort. Perhaps you are the only one from your college who made it to graduate school in the last couple of years. You still have plenty of time as a researcher to develop your own profile according to your interests. Don't be fooled by the impostor syndrome.

Dropping a bad frame:

Never mind what I said in the beginning. Science is not a competition -- at least it shouldn't be.

If you're doing research mainly to outcompete your colleagues, to improve your social status, to feel pleased with yourself: you're doing it wrong. Then you are extrinsically motivated, and this is neither sustainable nor does it make you happy. Have you ever delved into a problem and forgotten to drink or eat until the sun went down? Have you ever felt deeply frustrated because you just couldn't wrap your head around some strange finding, only to realize later that it completely makes sense when you look at it from a different angle? Experienced the enthusiasm that came with that discovery? Do you deeply feel that you have to understand this? Then you know the feeling of flow that really only comes from intrinsic motivation. Forget the competition; enjoy the game.

The vast majority of people working in academia are not the best. That is just a logical consequence of the fact that only one person can be the best and all others are not. Even if you relax the definitions by subdividing all fields and defining "the best" to be a group of individuals in a very specific sub-sub-sub-field rather than one individual, the group who is not the best must be significantly larger than the group who is the best in order for the concept of being the best to be meaningful.

A lot of work that needs to be done to move a field forward is just checking and re-checking, applying a result to different contexts, etc., etc. That does not require super intelligence, but is still vital. Identifying with my sub-field, and doing vital work to move that field forward is what motivates me.

Also remember that as you move up, you are entering an increasingly selective group: There is no shame in not being the most intelligent person of the world.

How do you come to terms with the fact that you might be doing research in what you love with an amazing mentor, but you’ll just never be the best in even your research community? How's that not discouraging?

The same way that a police officer comes to terms with the fact that they'll just never be the best police officer. Or a doctor. The same way that a business owner comes to terms with the fact that they'll just never be the richest person on the planet.

There's nothing to come to terms with, because "being the best" isn't the goal and "the best" isn't even a meaningful concept. People enter academia to study the subject that they love and to share their knowledge with others who want to know about it. None of that has anything to do with being "the best" because it's not a competitive pursuit. Sure, there are competitive aspects, such as getting jobs or studentships or funding, but job applications are competitive in any walk of life. And even if you ask ultra-competitive people, like athletes, they'll say that they're motivated by loving their job and wanting to be the best they can be. Almost nobody says, "I play sport X because I want to be world champion."