Has your wife talked to her doctor, OP? There are a variety of women's health things that could be contributing. Even if there isn't anything specifically contributing, some people's genitals just don't fit together that well. That may well be the case, and that's unfortunate, if so, but there isn't a lot you can do.
As for generally having an issue with not having sex often with the partner you agreed to monogamy with? All I can suggest is talking with your partner and trying to do better for each other. If that isn't working, but everything else in your relationship is solid, depending on the people, you could discuss opening the relationship so needs are being met. I wouldn't recommend that if there are any other things that aren't working so well in the relationship, though. Or if to you it's either your wife has more sex with you (even though she's saying she's in pain) or seriously thinking of cheating... maybe y'all shouldn't be together. I'm not saying that to shame you or to be mean. Sometimes people just aren't compatible. And it would suck, especially since you two have been together for so long, but I guess you have to decide what will work or won't, for you and your wife, eh?
Anyway, just some thoughts/rambles from a stranger on the internet. Take it with a grain of salt, or feel free to ignore
