Try a little bit of heavy cream. Broth could work also. The other thing that you should do is taste everything together. The sauce on its own might have too much lemon for your taste, but combined with the meat or fish that you are using it might be just what you want.

Chicken broth will help but take care not to let it ruin the consistency of the sauce. Or you can add the tiniest pinch of sugar. Stir it well, then taste. Don't overdo it. You just want it to take the sharp edge off. Not sure what else is in the sauce, but if there are fresh herbs, a bit more (but again, add judiciously, tasting after adding the tiniest bit) might help. Or a tiny bit of good prepared mustard. ;o)