Why can't I just ?

My boyfriend and I have been dating for a year, and just started having 6 months ago. I never could before, because he didn't last very long at the start. Now, he lasts longer, and I'm more sensitive. From the moment we start I feel like I'm about to, but it never happens. I'm stuck at the so-pleasurable-it's-almost-painful step throughout the entire session. Why can't i just ? Is he not lasting long enough? Is there something wrong with me?