Try stevia concentrate instead of sugar. There’s no standard for stevia, unfortunately, so it’s hit and miss as far as the various brands go. I recommend that you stay away from the powders, since stevia is so concentrated that they have to adulterate it with other substances to create volume, and my experience is that it makes the stevia bitter. I’ve tried a number of stevia liquid concentrates, and my favourite is Herbal Select. Don’t use Natures Harmony, as it has less than half the concentration as Herbal Select, and costs more.
Stevia isn’t heat-stable, but since you’re not baking, you can probably get away with adding the stevia just before you form the cookies and it should be okay.
If you’re unfamiliar with stevia, it’s an herb which is 400–600 times sweeter than sugar, has no calories, and – unlike artificial sweeteners – has no side effects. It was banned in the US by the FDA for years under pressure from Monsanto (the makers of aspartame) until the FDA was forced to allow distribution when it was revealed they had violated their own rules to ban it. However, it is still illegal in the US to refer to stevia as “sweet” and must be sold as an herbal supplement rather than a sweetener. It’s one of the more blatant examples of government corruption in the food business.