Trying to be Comfortable with Him

So I’ve had this boyfriend for over 6 months and I’ve known him for a long time, I love him and I feel like we’re kind of best friends too. We’ve just recently started to get intimate and he’s gone down on me before. I would love to reciprocate this but for some reason I can’t get comfortable with this intimacy. He’s told me he wants me to give him a handjob, and I said I would, but then later had a whole freak-out over it. He’s been understanding when we’ve talked about it before, but I’m scared this might push him away, and that’s the last thing I want. Every time this type of thing happens, I always think or even know I’m comfortable with the whole situation but something in the back of my mind is making me uncomfortable with it. I love my boyfriend and I’d do anything for him to be happy, but this has been difficult for me. Any advice?