Blowjobs

Ladies I need some advice. Recently my boyfriend and I have become sexually active. It's a lot of fun no complaints. But whenever he asks for a blowjob I just can't picture it. I mean I've given them before but I just can't get past the feeling of it or the end of it. I know he really wants one but he respects my opinion on it and doesn't force me to do anything I don't like. But I feel like maybe I've just never done it right?? I just don't want him to think I'll ever do it.