I want to try stuff with a girl 🙄

my boyfriend & i have been together for nearing 3 years. I’ve known I was bisexual since I was in 7th grade. I’ve haven’t gone past making out & rubbing with a girl. But I’ve recently been wanting to try. I am to scared to ask him if I can. Only because I am the only girl he has ever had sex with. I want to be honest but I’m also very horny & like the idea of secrets. I don’t know what to do.