If I was ‘sealed’ unworthily, does that make the ordinance null and void?
It depends on what you mean by null and void. If you mean null and void in the sense that you need to redo the sealing ordinance then no the sealing is not null and void. If you mean null and void in the sense that the blessing of the sealing are suspended and not in effect until you repent fully of your sins, and of the sin of partaking of a sacred temple ordinance unworthily then yes, yes it is.
You might be able to trick the Bishop and Stake President into thinking that you are worthy, but you will not trick the Lord. You will only heap up condemnation unto your own soul.
The way forward is repentance, to realize that what you did is wrong. Go to the bishop and confess, humble yourself before the Lord and ask for forgiveness. This is how you can claim the full blessing of the sealing and make in not be null and void.