Don't give up, get creative and let go. 
Personally, I can go from, 'Hmmm, I could have a quick one' to an actual quick one in two minutes, literally, and effortlessly. I remember how much hard work it used to be when I was first chasing an orgasm though - it can really make you feel like there's something wrong with you. I doubt there is; having trouble in the beginning, from what I hear, is actually the norm.
Based on what you shared here, I think - as some have already mentioned - you might just be pressuring yourself too much. You ask about ACHIEVING an orgasm, as though it were a high score on an exam. An orgasm should be pleasure, not a goal.
You could try playing with yourself - maybe two or three times a day, depending on your free time obviously - for a couple of days (maybe around ovulation or just before you get your period, because that's when your hormons are mixed up just the right way). Now, I don't mean masturbate to the point where you desperately NEED an orgasm, toes curled and all; just caress your body for a while to get in the mood. Don't go for the main erogenous zones UNLESS you feel the irresistible need to do it; there's millions of nerve endings elsewhere that can bring you much pleasure. Neck, the insides of your thighs and arms, around your lips, your ears, scalp - well it's a long list, but you get the idea. So just close your eyes, relax and explore, and let your mind wonder. Just keep in mind your goal is not a hard crashing orgasm, but a nice soft time to yourself. Just make your body hum, no more.
After a while, simply stop, go about your business. IF you truly need more, I expect you'll find yourself unwilling to stop at the end of one of those sessions, and you'll just slide off into an orgasm. If not, don't worry about it - most women's ability to orgasm develops over years. Just wait, you'll see
Also, do you do Kegel exercises?
And one more advice - might be a little bad taste in more ways than one, but I remember it helping me a lot when I was a teenager and I wanted my boyfriend to think he'd finally succeeded in satisfying me. It's MUCH easier to come when your bladder is full. Makes for a fireworks of an orgasm too, in my experience. Just be careful - if you're with a partner and they squeeze your abdomen, well, I imagine there might be trouble. More importantly, using this trick too often could potentially increase the risk of infections, especially when there's lot of penetration involved, so I would recommend moderation, and emptying your bladder and washing afterwards.
Everyone has really offered some great advice, so hopefully mine won't be too much of a repeat. There are so many ways to achieve a great orgasm, but I think the best is to start alone. Nearly everyone experiments as kids and teenagers, so I'd hazard a guess that most of us have had our first orgasm by ourselves. The response I've read to your question is that it really is the journey - the destination is AWESOME, but it's still fun to get there. Personally, if all I want is The Big O to relieve stress or go to sleep, my hand and a hot fantasy works great. Start with indirect stimulation of the whole vaginal area... rub your breasts and play with your nipples. I always find my breasts swell as I get more aroused. When things start getting warmer, move to more direct clitoral stimulation. Imagine how great it would be with another person doing those things. And, the great thing about fantasy is that you can fantasize about ANYBODY, and they can be or say anything you need them to. Trust me... before you know it, you'll find that SPOT, and you'll end up staying RIGHT THERE until it sends you over. And my other hand on my tits is definitely the extra stimulation to light me up like a Christmas tree.
Take your time, relax and let go...
