I'm 21 years old I've been sexually active since I can remember, and never was interested in getting or giving a blowjob. But now, I want to but don't know how to give oral sex in a way I know he will ejaculate. So I just open my mouth and then what?

My boyfriend and I have been dating for a good few years now, every time we have sex it feels like it's just for his pleasure. I really would like to be pleased as well, I mean the sex is nice.. don't get me wrong. But I don't think I've ever experienced an orgasm. I've taken advice from this site before, and I've tried masturbation on my own time, but it ends up taking me hours, and I just get frustrated. I've also discussed it with him, and we've experimented with so many things. The longest we've tried is three and a half hours. That's a little overwhelming and just plain annoying for the both of us. We've tried possibly every "extra" you can think of from biting, roleplaying, being sweet, being angry, to sucking, caressingly, and much more. Any type of contact down there just feels as casual as a massage, and the feeling never changes. What's my deal?