Girlfriend has been texting this other guy and he has been hitting on her?

I've been out of school for almost a year and we've been dating for almost two years. She's also out of school and this guy believe is a senior in high school. I've seen pictures of him. He's twice the size of me and has been hitting on her.

She gets mad at me and defends him saying there's nothing going on between them. Well it's been irratating me lately, so when she was at her physical therapy doctor. I went threw her phone reading their messages. Before I say what they've been talking about. I know it was wrong but I can't stop thinking about it, and IVE confronted her about it and like I said she defended him.

Ok these are the messages. He sends her kissy faces, tells her he loves her, says what you doing beautiful, cause her babe, says, and ask if I've been wondering who she's been texting all this time. I'm litterally about to pound this kid. I had no problem her texting him until I found out he was hitting on her that's when I got controlling. Please help what should I do?

I ask this question cause it's not only bugging the hell out of me, but there last text him: I love you😘😘😘

Her: I love you too😛