You don't say how long you've been trying at this, or how experienced you & your lover is... oral sex is an aquired skill, not something we're born knowing (no matter what the sex stories would have us believe... apologies to Pixie's Place here.) It could just be in your own mind you are holding back; it's happened to me on occasion, even though being sucked off is my favorite thing in the world. There is a little squemishness natural for both parties, fortunately the wife & I have both overcome most of this. My advice is to relax, let what happens happen, & remember; there's no such thing as a bad BJ, only great & fanFUCKINGtastic!!!