Be humble, be grateful. She found something that I (would hope) you like in a hobby that most parents would not encourage at all. It was very sweet of her to choose a game for you.
Appreciate it while you can. :) We stopped celebrating my birthday / christmas back when I was still in my teenage years. I just love my family too much to ask them to take on that extra financial burden.
This. Play it. Turn it in for 20 bucks get halo 4. I'd hope at 17 you can get some cash together to handle it. Not trying to be a dick about it.
Or sneak trade it in. And if she asks, which I doubt, just say it was a short game.
You're extremely fortunate that you have a mom that would do that. I never did. She did everything she could for me but couldn't afford games for me. Now that I'm older I realize how amazing she really was.
You sound like you got a reasonable head on your shoulders. do the right thing.
Just tell your mom thank you and I love the gift but you accidently got the wrong game. Then your mom will say "awww honey im sorry what game was it again?" Then you tell her Halo 4 then she will take it back, I used to do that to my mom sometimes, she dosent get offended she just returns it.