Orgasmic Difficulty
Before I even get into the advice portion, you need to know that this is EXTREMELY common! Most of the women I've slept with have not had orgasms because I came so fast! Just kidding. Sorta. But honestly, I've been with thousands of numerous girls who didn't come from sex or needed something extra. Don't be ashamed, okay? It's all good.
Now lets get into the advice part. Think of having an orgasm as unlocking a door. You need to make sure that there's nothing blocking the keyhole and you need to have the right key. The thing that blocks the hole (sorry, I didn't mean for this analogy to sound dirty) is pressure and anxiety. The right key is a matter of technique.
As far as relieving pressure and anxiety, some of that pressure is probably melting away as you read this post and discover that you're not a monster/mutant and actually fairly normal. The rest of it will dissolve as you adopt the right attitude and get your suitors to join you. The right attitude is not taking your Orgasm Quotient so seriously, giving yourself room to not have an orgasm and still enjoy other parts of sex (other sensations, the emotional aspects, etc.), and just having some fun. Lots of guys know that putting pressure on the girl to have an orgasm will backfire, but for those who don't, tell them that you just don't come from sex, but that they can go down on you for an hour afterwards. Don't let them take it personally and base their entire performance on whether you come; you can still enjoy sex if you don't come and having an orgasm doesn't ensure you've enjoyed the other aspects of sex. If they keep talking, just tell 'em to shut up and put out already.
Once you've removed the obstacles, you can address technique. For this, I'd suggest looking to your masturbation habits. What technique do you use when you masturbate? Hands? Penetration? Both? (Mouth?!) Try and adapt that technique to having sex. You can do some stuff yourself and guide the guy to do some of the other stuff. You don't have to come from it or calculate whether it'll lead to an orgasm down the line. Just ask if it feels good. If it does, continue. If you don't masturbate, then start. It'll help you get to know your body and feel more certain about how to navigate it.
You're a student learning about your body and mind. Maybe you have an orgasm, maybe not. If you experiment with your attitude and techniques like I suggested, the experience -- orgasm or not -- will feel better emotionally and physically, and that's a positive step.