If you can masturbate to orgasm relatively quickly, you are most unlikely
to have an ejaculation-related problem requiring treatment by a doctor. It's
also very unlikely that your girlfriend's vagina is too loose to make you come. Vaginas are extremely
elastic orifices, and unless your penis is extremely small - as in small enough to have made you take it
to a doctor - you should be able to fit together smoothly and effectively.
Perhaps you have only recently given the gift of your virginity, or perhaps
you haven't shared sex for a long time, and perhaps you (either your mind or your body) have
never learned, or forgotten, about having an orgasm through penetrative sex.
I know it is widely believed that if one masturbates in only one particular way over a long period of time it can become the only easy
way one can reach orgasm, so perhaps this is what is happening to you.
If you and your partner are like most couples who speak about this problem,
your partner will find this a lot more psychologically difficult to cope with
than you will. She will be feeling guilty for not being able to "make you
come" and sad and worried that you are not coming, or not coming easily
because you don't really love her, or don't desire her, or because her
hair's the wrong colour or generally laying the self blame on pretty thickly.
You'll need to be sensitive of her feelings while dealing with this, and at
the same time demand she be sensitive of yours. Just saying "It doesn't
matter" won't tell her insides anything. You'll really have to work hard
to get the message across, in all likelihood.
The problem is likely to resolve itself given time and familiarity with
your partner. And in the meantime, you and your partner could consider finishing the
love making session in a way that combines your manual stimulation of yourself
with the loving touch of your partner.
cf When sex hurts (men)
It's a very pleasurable experience in itself and sort of a way of screaming triumphantly at the universe "Hah! You wouldn't let me come
in her but did I cry or get mad? NO! I did it like THIS and I LIKED it! So take THAT!!"