Here's the good news:
you're not alone. One of the many things they don't tell you about sex when you're growing up is that it's not
a forgone conclusion that you will always enjoy it.
It's hard to find the right psychological balance for sex in a steady monogamous relationship. If you're used to being promiscuous (or at least easy going), you're faced with the challenge of upping the level of intimacy in the sack beyond that of the casual encounter. If you come from a background of steady long term sexual relationships, you have to adjust to the preferences and stimulae of a new partner and un-learn many things you've learned in the past. If you are of a more traditional, less sexually active persuasion, you're faced with a mountain of inhibitions and preconceptions to get rid of. It gets you coming, going and standing still.
The best thing you can probably do for yourself and your partner right now is to avoid obsessing about it - guilt, fear, frustration, these are al the enemies of a good time in bed. High expectations have a lot to answer for as well. You can't relax if you're constantly stressing about not being relaxed enough! I know this is easier said than done, but when you're ready to make the decision it will come very naturally.
Another thing I would recommend you try is talking to other women - friends, sisters, and best of all your mother or another experienced older woman. It's amazing how liberating the realisations that your problem traverses generations and is not something new that is punishing you for past promiscuity can be.
As a side note, and of course only if you feel comfortable and don't have a history of substance abuse, a joint or three before bed can't hurt. Marijuana relaxes not only the mind, but the body as well. Loose muscles and good circulation in the right places can do miracles of effortless pleasure. I wouldn't recommend alcohol - it's a downer, and as the hormonal reaction in the body after orgasm plays havoc with the emotions anyway, this can prove counterproductive. I'm reliably informed that sex on E is the absolute best, but I've never tried it myself.
Please understand this is not to be used as a crutch - it's just a good way of helping along that one crucial time which bursts the dam and makes everything suddenly much easier.
Go easy on yourself. This is the best advice for life in a nutshell.