In the sentence, the verb try is in the present tense. We use the present tense to express actions that are repeated or habitual, e.g., I work every day / I try very hard. So, I try to be a nice person means that I always make an effort (try) every day / all the time to be a nice person. The sentence could mean that I used to be a bad person and now I try every day to be nice, but 98% of the time, the sentence is an affirmative statement that says I am always trying and does not refer to the my behaviour in the past...

Your interpretation is possible. However, it is common for someone to say this if they see themselves as a nice person but also acknowledge their failings. "I try" in this case is a way of the speaker saying something good about himself while still showing humility.