@PvtPancake7,
Time for you two to take a break.
Suspend your relationship. Make it nonexclusive again.
She can see other people (including him, I might add, if she so chooses).
This also means that you can see other people. So do so! Go out and have your life and have fun. Do NOT sit at home and pine your hours away. You can have fun and not be mean but also not looking for anything lasting or love or whatever.
But put a hard deadline on this, too. Say, two months? And then you both reassess. Want to stay together? Then some things will have to change. She cannot turn her feelings on and off like a switch (and it's good that she's been honest with you), but if she still has feelings for him then the mature thing will be to let you go as she cannot give you her fullest attentions and that's not fair to you.
In the meantime, you might have met someone you like better. That can happen. It's okay. The vast, vast majority of us do not stay with our first loves forever.
And please be responsible and use condoms.