I too am very difficult to get off. I highly recommend porn and fantasy, and barring that, explore your body and try to figure out a toy that will help

Read some Emily Nagosky. She explains everything beautifully ('cause I can come in under 3 minutes it was hard for me to understand women who have difficulties - I got it when I read her):

The dual control of sexual response breaks the sexual
response system into two parts:
Sexual Excitation System (SES). SES is the system
that responds to sexually relevant stimuli in the environment,
from visual stimuli to tactile stimuli and
everything in between.
Sexual Inhibition System (SIS). SIS is the brakes
system. Research so far shows that it’s likely there are
two different SIS systems, one that responds to fear of
performance failure (erectile dysfunction, premature
ejaculation, etc.) and another that responds to fear
of performance consequences (STI transmission, unwanted
pregnancy, social consequences).
Both men and women have both of these systems.
Sexual arousal is a process of “disinhibition”—it’s not
so much “getting turned on” as it is “turning off the
offs.” So you can think about arousal as two equally
important processes: providing gradually increasing
stimulation for the SES, and getting rid of everything
the SIS might respond to, which includes both physical
and emotional risks. It’s like putting your foot on the
gas and taking it off the brakes.
On average, women tend to have more SIS (more sensitive
brakes) and less SES (less sensitive gas) compared
to men, though there is lots of variability. This means
that, in general, women require more stimulation to
become aroused, and that women are more sensitive
to all kinds of threats—including physical, emotional,
and social.
If you’re a woman who has trouble having an orgasm or
always takes an hour to have an orgasm, you might be a
woman with relatively low levels of SES and high levels
of SIS. For you, it takes a lot of stimulation to activate
SES sufficiently to generate a really high level of sexual
tension, and it only takes a little bit of anxiety or stress
for your body to hit the brakes. If g-spot stimulation is
pleasurable for you, it might be a good way to explore
your orgasmic potential, because it vastly increases the
intensity of stimulation.
Orgasm happens when you generate a sufficient level
of sexual tension in your body to cross a threshold,
when all that tension releases explosively. For those
familiar with the sensations of clitoral orgasm, you
might recognize these signs: erratic heart rate and
breathing, muscle spasms, waves of pleasure, and the
characteristic pulsing of the pubococcygeal muscle at
the mouth of the vagina. For women, the whole thing
takes something like 10 seconds. For men, it’s more like 5.

You can get her Female Orgasm e-book for free if you go to GoodInBed.com and complete a short survey on what you think about monogamy.

I have had the same problem until recently. I still haven't gotten off from sex alone, but I have from a lot of oral/manual stimulation but I had to be EXTREMELY relaxed for it to happen. Try drinking a glass of wine or something beforehand, just enough to help you wind down. Try lots of foreplay and kissing and rubbing too -- that seems to help!