A few possibilities.
Have one or both of you become complacent in your relationship? Meaning, do you just do the same old thing, day after day, week after week, year after year with no new things added into the mix. Not just sex, but all of the other things that you do, like going out to movies or plays or lectures (things that are exciting and new to both of you) and trying new foods and places, or planning vacations to places you’ve never been before. Sometimes if you simply get home from work, throw the same old frozen food into the microwave, drink the same beer, watch the same TV shows, brush your teeth and go to be and an attempt to have the same old sex during Leno or Letterman, it gets old real fast, especially if either one of you is shy, or quiet, or simply doesn’t have any other interests or passions in anything. Could this be part of your problem?
Another possibility is that you have come to the realization (or you haven’t quite yet come to the realization or don’t want to admit it) that although your partner is a nice guy, he may not be the right guy for you. Is this the situation you’re dealing with?
Some women experience changes in their libido due to hormonal problems or other medical problems that they might not be aware of. Have you gone to your doctor and your gynocologist to discuss the possibility that your hormones may not be on track or that you might have some other underlying physical problem or even depression or anxiety, which can also cause you to lose interest in sex.
Is there the slightest possibility, that you might be gay or bisexual, but have never really thought about it, or you were afraid to think about it, or you were disgusted to think about it? Sometimes people go into a relationship, at a young age, assuming that they are straight because that it what is expected of them, but later on down the line, when they’ve been exposed to other things and people, they might come to the conclusion that being a straight female in a relationship with a man is not exactly what feels right for her anymore. This happens to males too. Could this be something that is happening?
Is there the possibility that you have fallen in love with, or developed a crush on another fellow, but you simply don’t want to admit it to yourself, because of guilt or knowing that the other fellow is not available to you for whatever reason? If it turns out that this is the problem, then you either need to seek some couples counseling with your current boyfriend and let him know the problem and see if you can work through it, or decide that your only option is to break up.
If you seriously can’t figure out what the problem is, you should first talk to your doctor and gynocologist, and then make an appointment with a therapist who can advise you whether it’s worth seeking couples counseling with your boyfriend. Don’t leave your boyfriend in the dark, though. Let him know that you’re struggling with this situation and that you don’t want to hurt him, but you have to do something. Find out what he thinks. He might be mad at you, or he might not understand that there is a real problem. But you need to get to the bottom of it soon, so you can make a good decision for both of you.
Good luck : ) And let us know what happens.