Most Helpful Guy
Grab a hold of your balls that have shriveled up into your body and fucking stand up for yourself. You don't want to leave her? The other option is constantly thinking in the back of your mind that she's cheating on you and getting dick pics on SnapChat from other guy (s).
You're under 18, not the end of you life. You'll have plenty of times to meet better girls than her, let's face it man cheaters are worse than trash. Do yourself a favor and call her out on it. The nails are pretty much in the coffin it seems, so unless she has a lot of proof showing she is not cheating you need to break up with her.