I wonder if part of the problem is psychological? I know that sometimes I go through weird phases where I keep being on the way to orgasm then I have a sort of mental block...like I suddenly get self-conscious or something and completely snap out of just being totally within the physicality of it all, and then it takes me ages to get back to where I was. I can go for months where this doesn't happen, then a week or so where it will happen every time for no apparent reason.
I wondered about that because you said sometimes you got worried during oral sex that your DP was getting bored? If your thought patterns during sex are similar, this isn't going to help you orgasm, I'd have thought. Find a position that feels good and stimulates your clitoris (agree with previous posts that a cross between missionary and sideways is often really good) don't let your thoughts wander to whether or not your DP is bored or whether or not you will orgasm, just close your eyes and focus on the spark of pleasure you're feeling, however small, and see what happens? It's not a race...speeding up before you're ready to speed up is fatal! And 10-15 minutes to get to orgasm, even through oral sex, doesn't seem like that long to me, even if it is slower than DIY.