Let's deal with the idea that your girl makes love with someone else. How does that make you feel?
Jealousy, anger, hate, you wonder how does she look, what does she do, you would like to see it for yourself to prove you that you are not the one that moves her heart, to convince yourself that there's somebody else with the ability to take her clothes off and love her, somebody else that can kiss her and make her happy.
But you don't dare, do you?
The idea of someone else making her moan and maybe even scream kills something inside you, and you keep the hope that she's going to bring you back to life by saying I love you and all that awesome stuff, but it's not gonna work out because in fact there's someone else in her life and even if she loves you, you'll never be the one.
Now tell me... Can you deal with that?
Is your love strong enough to go through her life style?
Don't lie to her, tell her the truth, tell her that you know you are not the only one and work it from there.
But more important... Don't put your hopes on having her all for you!