Can I go and cancel my foreign expert certificate?

It's possible that you can cancel the FEC by yourself, but you must get the help of your employer. For what I know, when you cancel it, you need to submit an official letter from your company which has a common seal of the company. If you can't get this document, it's hard to cancel your FEC. And if you cancel this FEC before its expiry date, you should submit a detailed statement.