I am a nymphomaniac

Do you want s** more often than your boyfriend? There are girls who have stronger s** drives than a lot of guys.
If you are not satisfied with the amount of s** you get in your relationship, perhaps you are not well matched as a couple. I cannot say about that, you will have to decide.
If you really feel this is a problem there is always s** addicts anonymous. You can at least "share notes" with others and see if you really have as big a problem as you believe you do.
It might be that you simply have a high s** drive, and you need to find someone else who does also. If you stay in a poorly matched relationship, you will be frustrated and tempted to have affairs outside the relationship.

You'd get better help if you say more about "I think I have a problem"
For instance, is the problem how much time you spend thinking about s**? Is it creating tension of fear within your relationship ? Do you have urges to act out that would damage or destroy your relationships or lifestyle ?

Not much I can help you. But a word of advice:
You're not the only one.

I've always felt that a good definition of addiction is when it interferes with other aspects of your life. For example, have you ever called in sick to work so you could m*********? Have you missed important social or family events? Do you neglect household chores or even personal hygeine? If so then you may have a problem.