Maybe it is as though you are stranded somewhere without sherry. It is not your fault that no sherry is available; the waiters suggest port, but port is not what you want. It's not your fault. You want sherry. But the choices are clear: Port is all you'll get. Such is life.
Or maybe it's worse: It's not that they're out of sherry. They're just not selling it to you anymore. Oh, that's sad. But it is a fact of aging, however painful: We do pass out of youth and its automatic privileges.
All this is just about exploring the shape of this thing so you can get comfortable with it, knowing what it is: It's just a change that has happened. It's just the way you happen to be right now.

Next comes the question of what to do about it in the real world.
I do think, though, that the best thing to do is to be upfront about this. Otherwise you are not really acting in good faith; he thinks you want sex but you don't. Instead, you are having sex in return for something else. That's not going to lead to happiness.
Yes, it may ruin some dates. But it may also have unexpected benefits: Once you are free to stop pretending, you may find that you are having more fun. You might even start enjoying sex again, once you are free to not enjoy it.
good advice By Cary Tennis, and unexpected...

Salon: since you asked: I enjoy men until we get to the bedroom -
It seems that my mind loves making love but my body hates it and the sex act itself deflates and depresses me. While I started out my sex life with a fairly high drive, I've found that for about the past 10 years (since I hit about 35) sex is just not as interesting to me as it used to be.