Я думаю, первое.

This doesn't make sense. You can't be sure of the outcome or result of your efforts, but you CAN be sure that you're doing the best you can since we all know our limitations. When would anyone say this?

Actually here, a boy is pretty confident that he can be sure about his results, because he is a super hero

https://getyarn.io/yarn-clip/005dbf21-373d-4a02-b0de-3dd0c10d326f

Ok! His mother wants him to excel and win at something, he just wants to “do the best that he can do” even if he loses. In other words, “Why can’t I just do my best and if I lose, it’s ok?” Instead of aiming for the best and not accepting defeat.

Оk, now I'm trying understand how would you say: Почему мне нельзя делать то, что у меня получается лучше всего?.

You would say: “Why can’t I do what I do best?”

thank you a lot😁🤟

*THANKS a lot 😉

you’re very welcome