Many men find it difficult to climax while being fellated. You may have a hang-up about coming in your girlfriend's mouth. You may need more friction. Add a hand (yours or hers), or one of you can add manual stimulation elsewhere -- touching, rubbing, squeezing, etc. -- to assist the process. You may need a better view -- so that you can see, and therefore believe, that it is actually happening to you. Do it in front of a mirror or in better light or standing so that you can look down on the action. Or perhaps you need to be more active. (During intercourse most guys get carried away by the thrusting action -- it could be that you are simply doing it with an unfamiliar rhythm.) Finally, try switching the order, using intercourse as foreplay.