Am I Broken?

Ally
Right, so I have been masterbating and such for a few years, and just started having sex with my boyfriend less than a month ago; we had been doing oral for a while before that. But, the thing is, I can never cum! Not even when it's just me! 
I know it's not him, and I never feel stressed or anxious or anything, so what is it? If anyone can help, that'd be great!