hard to orgasm with new guy

I'm a girl and I've never really had problems reaching orgasm with PIV or touching myself, and normally can have multiple orgasms.

I've starting seeing this new guy and all of a sudden I'm having problems. I can now only come sometimes and only from 1 position. He's great and keeps asking me to tell him what I like. However, I think the main problem is that I am intimidated by the number of people he's been with and comparing my relative lack of experience to his. In the past I've always been the more experienced of my sexual partners, so this is weird for me. When he asks I kind of shy away from talking because I'm embarrassed and I don't know why I can't just act normal.

I'm also sensitive about the fact he's more athletic than me, so I get self conscious about my lack of stamina (not how I look). I've been trying to do more exercise, but this difficulty orgasming is becoming more of a psychological problem for me.

I haven't told this guy about my anxieties, as I don't want to put him off or anything...can anyone please help talk me out of thinking like this and being so intimidated?

Thanks