My Girlfriend Admitted She Has A Side Guy...

I've been in a relationship with this girl for nearly a year, but I've only recently started to truly enjoy it. Over the past couple of months, I've noticed she's been frequently in touch with another guy. When I confronted her, she revealed that she’s seeing both of us. She told me she loves us both, doesn’t want to lose either of us, and will choose the one who wins her heart to marry. I deeply care for her and am willing to do anything, but I'm concerned because she started dating the other guy before me.