How do i know if i'm a lesbian? I love with my partner but confused coz i find some guys sexually attractive?

I'm 27 and had a boyfriend for 9 years before i found my girlfriend. I fell in love with her immediately and we've now been together for 3 years. Last year i met this guy who i fell for, and ended up cheating on my girl. I feel terrible about hurting her but i was so confused at where i should be. I love her so much. Please help me figure this out and share with me if anything similar has happened to you. Or am I the only one???