You're not alone. Many women are in a relationship with a partner that has premature ejaculation (PE). In fact, it's one of the more common issues I tackle with clients I coach.
PE is generally caused by anxiety or excitement. Sometimes, the stage for PE is set during adolescence. The classic scenario: A boy rushes to get off so he doesn't "get caught" masturbating (by a parent, sibling or someone else). Pretty soon this lightning speed pleasure session becomes his routine. Then, when he's intimate with a partner he can't hold back, and this pattern can continue for years. It could also be that you really turn him on. Not to mention, he's likely psyched to get laid. Of course, other mental culprits and physical causes can be to blame. These include stress and giving in to the pleasurable sensations, rather than having an awareness of the levels of arousal and how to navigate them. Ask your boyfriend if any of these are the cause, or if it's something else. If not, don't hold it against him if he doesn't have a clue.
A possible solution to try: If you don't already, use a latex condom with added climax control lubricant like Extended Pleasure by Trojan or Performax by Durex to help him last longer.
Many AASECT-certified sex experts--like myself--can help guys with PE. Our role is to gain insight into the situation and help retrain their minds and bodies through conversation and activities that are done alone or with a partner at home. These techniques often include understanding how to identify sensations leading up to the "point of no return" (the time when a guy realizes he's about to cum and ejaculation is inevitable) and developing the ability to control his arousal.
One tactic, called the stop-start method, requires him to stop the in-and-out motion before he has the feeling he is going to climax, and start once it subsides. Another approach, called the squeeze method, works by applying pressure at the base of his shaft or the corona (ridge under the head of the penis) with the thumb and index finger before climax is imminent.
Many of these tricks can be found online, but truly understanding the process, tweaking it to be relevant to his needs, putting it into practice from a pleasurable perspective rather than a clinical technique, and monitoring progress that leads to change generally works best when discussed with a qualified expert. An MD can prescribe medications, although this is best as a last resort.
With all that said, don't forget to prioritize your own pleasure. You can still reach the Big-O by practicing tons of foreplay. The next time you're together, encourage your guy to lavish you with touch by asking for what you want. You can say something like, "Let's see how turned on I can get from your amazing oral," or lead his hand to your vulva and tell him, "I'd love if you use your magic fingers to make me cum." The point is to welcome the attention, enjoy the sensations and have your orgasm before you and your guy have intercourse.
With practice and patience you'll have your orgasms and your boyfriend will learn to extend his pleasure potential, leading you to an awesome sex life that hopefully causes you both to lose track of time.