Is it me?

So I've been dating my bf for 3 months and we've had sex and I've given him blowjobs but every time we do something he never cums. I've been with a few guys and I've never had this problem. My bf tells me it's not me he just gets nervous and can't he's only been with 2 girls besides me and he tells me every time he can tell it upsets me that it's not my fault and I'm not doing anything wrong he just gets nervous. Anybody have any advice or had this problem before? And do you think it's my fault because sometimes I really think it is even though he says it isn't.