So I've dated this girl for a year. I'm 20 and she is 17 will turn 18 in June. She always said she was in love with me and could see a future with me. I always treated her right and always appreciated her, she even told me. Her friends and family even told her that too. Of course we had small little arguments from time to time but what relationship doesn't. Well in second week of April she decided we should break up. She said her feelings have changed but she cares about me. I found out she met a guy 2 days before that and was texting a guy. The guy is 23 and has a kid?? Found out she hung out with the guy and she started bawling and felt bad for doing it. She said she wanted to get back together and told the guy she was getting back with me. Then two days later she wanted to break up.She said this guy is nice and treats her right. She said he told her she should be appreciated more and acts like he cares about her, which is BS. Its exactly how I treated her in the beginning. So we broke up and I tried talking to her a little for two weeks which was stupid. She's been seeing this guy for about a month now and I've went no contact the past week. Told her I care about her a lot and that ill leave her alone. I worry about her everyday being with this guy. I want her back. Is in a rebound relationship? Will she come back?

No one can know if someone will go back to someone.
But I can tell you this isn't rebound on her part. She met him while she was your girlfriend! Rebound is getting involved with someone too soon after being dumped.
The key fact here is her age. She's still young enough to change her mind about people at a moment's notice - fickle.
At least you can pat yourself on the back for doing all the right things (except for getting involved with her in the first place). Stay NC and mean it. She may be gone for weeks, months, or forever. You just don't know, so have to act as though she's really gone.

When you get dumped for another you disappear, heal, and move on. Her feelings changed and whether it's a rebound or not doesn't matter as she was probably thinking of dumping you long before she told you, and was already "talking to him".

Let no more contact work to heal you my friend. Breakups always suck, but not worth giving up your dignity and self respect.