The requirement for the conditional adverbial clause introduced by if is that the main clause is an imperative sentence in the future tense or when there is a modal verb, the subordinate clause is in the future tense

Simply speaking, the subject is present

nothing is impossible if you try your best

nothing is impossible is the subject of the imperative sentence, which is equivalent to everything is possible (will happen)

According to the subject-subordinate principle, if follows You try your best. Naturally, use the present tense.

PS In fact, spoken English and famous sayings are not very sensitive to tense requirements.