URGENT! PLEASE HELP!

I've written a fanfic in first and currently writing in third, so maybe I could help a bit in solidifying what you could get from both?

First is a pretty good perspective to start off for newer writers, in my opinion, because you could really concentrate on one person than multiple ones at once. As you said, the story belongs to the narrator more if s/he's the one talking. The major downside to this, though, is what delphin said. I could also say a con about first is the fact that you would need to balance the plot and his thoughts/interpretations. Also, don't be afraid to correct him in-story. Because the reader will be with him all the time and inside his head, what he says about the world can come out like as though you personally think this, as first-person is a more personal perspective. It's also hard to be more subtle in first-person with your narrator, but I'm not condemning this, honestly.

Third is where you can be subtle about the character's actions better than first. There is a third-person omniscient, but this would apply to all characters (I think). Yeah, the story wouldn't belong to Finn as much as first, but it's a good thing he's the sole protagonist. If you concentrate the perspective only on him, it brings out the illusion that the story solely belongs to him. (Think HP without the intro where we are given some details about the situation to come; it would seem that the story solely belongs to Harry because we are not given the details of anyone else but him).

Before you jump into this, I think it would be best to read through some short stories or books with flashbacks in first and third. Just to see if you would like to do the same.

Honestly, here, I would go for flow. Shifting between first to third might rattle the reader, especially if they encounter their first switch. I'll agree with delphin and say it's probably best to stick with one or the other, because I have encountered a book that did a switch of perspective and it jarred me at first and I disliked it, but don't pay attention to me.