Judging my some of the stuff you have said. I have a theory. I theory based on some of my own experiences (Minus the sleeping with someone else part)
Okay look at it this way. Your lady had this orgasm with a guy she spent 20 minutes in a car with, yet you often lavish her with attention, hour long oral sessions etc and nothing happens...
Okay here is my theory based on that above paragraph...
For the most part, women get turned on in a different way to men. (I reiterate: For the most part) Men are very visual creatures and also are stimulated a lot easier with physical touches. Women on the other hand...It is different. I would say that most of a womans arousal level is based on whether or not she is turned on mentally. Direct stimulation to our body parts, and visual stimulation is all good for us, but the difference between an okay session and a mind blowing "omg I just fainted" session, often lies in how you have managed to get our mind working.
Judging by what you have said (She orgasmed in a car with this guy, but not you, although in the early days she seemed closer) I am totally hedging my bets that you have yourself a woman who is very much like me, in that mental stimulation is what will make or break her arousal. The reason I think this is because...she orgasmed with this guy. Why? maybe because it FELT so naughty to her, maybe it felt taboo, naughty, secretive, exciting, frantic, lustful, hot....(All of these are FEELINGS, see? Rather than actual physical stimulation, they are what makes her brain tick, what gets her off most.)
So what do I suggest? Mixing it up. Trying new things to stimulate her mind and get her thinking. There are so so many ways to do this that the only limit is your imagination. Do not focus on giving her pleasure physically, rather, try to get her chomping at the bit before you even make it to the bedroom by teasing her with words, thoughts, feelings you have, talk about fantasies etc. Another tip is to try to let her know how absolutely amazingly horny she makes you (but not by just saying "I am horny" but rather by being more specific (eg: When you bend over and those jeans tightened around your arse I could not stop imagining....etc. I know that nothing turns me on more than the idea throughout the day that my partner is itching to rip my clothes off.
I have a feeling if you hit on this ladies mental thrills, you will be wowing her in no time.
I am a lady who suffers the same. I often struggle to reach orgasm when it is just a case of us falling into bed, hands wander and then 5 mins later he is between my legs and the room is silent and I am scanning my brain to bring back some hot fantasy to help me along. There is nothing wrong with my partners technique and he is more than willing, like you, to go at it for as long as it takes but...its a struggle. I need to FEEL the naughty, the kinky, etc. Massages, candles etc are all good but nothing on what you could do to a woman with the power of stimulating her mind. trust me :D
Good luck xxx