The condition that you are describing sounds like orgasmic dysfunction. During sex, sometimes a man experiences inhibited sexual excitement and/or loses the ability to orgasm which is called "anorgasmia".

A complete medical history and physical examination needs to be done. Results are almost always normal. If the problem began after you started taking a medication, this should be discussed with the doctor who prescribed the drug. Treatment depends on the cause of the sexual dysfunction. Medical causes that are reversible or treatable are usually managed medically or surgically. Physical therapy and mechanical aides may prove helpful for some people experiencing sexual dysfunction due to physical illnesses, conditions, or disabilities and a qualified specialist in sex therapy may be helpful.

I'd suggest that you get a physical exam first and discuss the situation with your doctor. At least you'll feel better knowing that you're taking the right steps towards solving the problem.

you are trying too hard. dont think about it. just relax and focus on your girlfriend and it will happen. obviously you can have an orgasm as you said you can by masturbating so its all a mental block. just relax.
by the way, you are too young to be having sex...lol

jackking,
Although I would not discount going to see your doctor if you feel the need, but you may just need to relax a bit when having sex or getting a blowjob or handjob from your gf. Also, although this may not be the problem, a guy can desensitize his penis if there is excessive masturbation or if the masturbation technique used is too harsh (or if it is always done without the use of a lubricant).

You need to take care of your penis, in more ways than one, so that you can cum when you are with your gf.

Wendy