I don't think this has been asked, serious question though -- do you have any issue when you're on your own or can you complete the deed as quickly as you want? If it's not an issue on your own, I suggest you learn exactly how to give her an orgasm. Work on getting her off, after that she'll likely just want you to get off so things can be done. That being the case, and if you are are realizing you're not going to be able to finish inside of her, I wouldn't have any shame about finishing yourself on her -- chest, stomach, hell maybe even face if she's into such things (best to check first!). I've had similar issues, but never have any issues on my own, so the above method will usually do the trick if I run into difficulties. Give it a shot (so to speak) and save both of you some pain.