People who think this way believe that the world, and all the goodness in it, is like a giant pie. For a person's slice to get bigger, somebody's slice has to necessarily get smaller. You have to remember that if you want to achieve the best, ‘the best’ is largely defined by you. The outcome that you get out of any effort is ultimately going to be judged by you.
When you say that the best is already taken by others you're just giving yourself an excuse not to try. You are assuming that there's only a fixed amount of "the best things in life." That's not true. You may come in third place and still think you did your best. That is good enough.
Stop robbing yourself of opportunities for victory by refusing to even try. Ultimately, whether you came in the first place or not, what matters most, at the end of the day, all boils down to whether you selected a vision for yourself and did everything you could to achieve that vision.
That's going to be the measure that you're going to have to live with. That's how you determine ‘the best.’ Sadly, thinking that ‘the best has already been taken is just an excuse to not even try.