Relationship anxiety

I've been with me boyfriend for 1 year 7 months now and it's been the most amazing time of my life. However before we got together, I had been in relationships before and has had sexual experience, whereas my boyfriend hasn't. I constantly think in the back of my mind that he wants other girls, and wants to experience what sex is like with other people and it scares the hell out of me the thought of him and another girl. It takes over my everyday thoughts and I don't know what to do anymore