Would the nuance of this sentence change if I omit "on" and just say "early in life" here?

Can I also say "live on a city"?

"Live on a city" cannot be used here. "On" implies that something is on top of or on the surface of something, so "living on a city" implies that one is living on top of a city. "In" is best for this context. Ex: I really enjoy living in New York City. The city is dynamic and there's always lots to discover. Ex: The old man lived on top of a huge tree, in a house he built with his own hands.

Does the expression "I didn't mean to" considered a complete sentence? Or is something omitted after the word "to"?

It is considered a complete sentence when said in reference to something else! So within the context, the meaning is implied. You can also continue the sentence if you feel it is appropriate. Ex: A: "You made me so angry!" B: "Sorry, I didn't mean to." Ex: "I'm sorry! I didn't mean to make you angry."

Does "as long as" have to do something with distance? What's it mean?

There are two main uses for "as long as." Firstly, it can mean "provided that." In this context, "as long as it's preloaded with information" can be understood to mean "provided that it's preloaded with information." Ex: As long as you get here, I don't care when you arrive. Ex: As long as you finish this project, you will be able to graduate. In the second use, it means "during the whole time that." It's also sometimes preceded by "for." Ex: The security guy here is super nice. He's been here as long as I can remember. Ex: For as long as I am able to, I will exercise every day.

What is the meaning of "boo" and "fly girl"?

"Boo" is a term of endearment given to a significant other. It is slang for "sweetheart" or "honey". This word is more common to use in American English. Ex: He is my boo. "Fly girl" is another slang term that refers to a girl who is really cool and unique. This phrase is not super common. Ex: She doesn't care about what people think, she's a fly girl.

What does “pretty” mean here?

In this context, "pretty" is an adverb, and it means "somewhat", "moderately", or "fairly", but not extremely. Ex: It's pretty cold outside today - not too cold but colder than yesterday. Ex: I got 90 points on the exam, I think I did pretty good.