Upvote:4
You should contact the consulate of the country that issued your visa and ask them to cancel it. Make sure they put a "cancelled without prejudice" stamp. If they do, you shouldn't have problems getting Schengen visas in the future.
If your visa is about to expire, consulates will have no problem cancelling it. But your circumstances are different and very unusual. It's hard to tell whether the consulate will ask you for a reason (most likely they will) and agree to cancel you visa (it's likely they will cancel your visa).