First, if you are GST/HST registered, you must charge and collect GST/HST on your sales, and remit it to the CRA (Revenu Québec, in Quebec). You might be registered because in a past year you crossed the $30,000 threshold, and you have not cancelled your registration. Or you might have chosen to register even as a small supplier, so that you can claim input tax credits on your purchases (and because your customers are businesses that do not mind if you charge them GST/HST). Either way, once you’re registered you are “in the system” and the $30,000 threshold is not relevant. You must charge GST/HST on all your sales (unless they are exempt or “zero-rated”).
Second, when counting the $30,000 threshold, you must total up not only your own sales, but also those of other persons with whom you are “associated”. This includes any corporation you control; as well as any partnership from which you (and associated persons) are entitled to more than half the profits; and certain trusts. Your spouse is not normally “associated” with you.