I am writing a persuasive essay and this is my first sentence (see below) but on my word processor when i go to check spelling and grammar it always underlines themselves and tells me to change it to they (but that doesn't sound right)... I really would appreciate any help possible.
I as a person believe that everyone needs to discover themselves to live life to the fullest.
Well, changing it to "they" would be wrong.
I would delete "as a person" as unnecessary. Of course you're a person.
Although we do use "their" or "them" as a gender-neutral, singular pronoun, you can also change "everyone" to "people" to avoid any mismatch in number.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.