It sounds like you are very anxious when having sex with her. You put a lot of importance on the end result, instead of focusing on how good you feel.
First step is try to relax. Don't put so much pressure on yourself to "perform". Pay attention to how you feel...enjoy the moment. Don't worry about the outcome. Sex really is not about the orgasm...but for some reason everyone seems to think this is the goal. Enjoy her, the moment, what you feel. Keep in touch with your sensations. Don't try to "run past them just so you can finish". Enjoy the lovemaking, the pleasure, your company. If you take your focus off the end result and be in the moment, pay attention to your sensations, your feelings, the intimacy, yhen everything else will take care of itself.

Read up on Sex Therapy. It teaches Sensate Focus a series of specific exercises for couples which encourages each partner to take turns paying increased attention to their own senses.
These exercises were originally developed by Masters and Johnson to assist couples experiencing sexual problems, but can be used for variety and to heighten personal awareness with any couple.

Read up on this and try it. It will help you get in touch with yourself and to relax and enjoy the moment. The more pressure you put on yourself to perform, the more difficult it will become. Relax, have fun and good luck.

There could be numerous reasons contributing to her inability to get you off, but judging by your age and assuming you are a healthy male i would narrow it down to two reasons. 1. You are really nervous when you and your gf make out which dampens your sexual response or 2. You have been accustomed to a certain technique to get you off, either due to masturbation or previous sexual encounters. You mentioned that you can get yourself off and this is what may have lead to your inability to orgasm with you gf. What i suggest you do is, the next time you masturbate take note as to what you like such as position (standing, lying down) and weather you enjoy things like playing with your nuts while masturbating or clutching your ass before orgasm whatever. Believe me, if you have any "habits" i suggest you tell your gf what you like and let her stimulate you under these same circumstances. Or you could try masurbating in a different style to break old habits, takes time but works!!
GOOD LUCK

that is scary.. its the exact same with me. i'm 18 and the exact same problem is happenin to me. i was wonderin was it just me or what the deal was.. i need help on the subject and i dunno where to turn to. if any1 can help, please do