“I have the joint meeting for my mediation tomorrow and I don’t want to go. What will happen if I don’t?”
At such short notice you will not be released from having to pay your share of the mediator’s fee and other charges such the venue hire and the services of an ADR provider. If there are proceedings running and the mediation has been set up in response to a court order (a so-called Churchill Order), you will be breaching that order if you do not attend. The court may then strike out your claim or defence under its case management powers. If so, any fees paid to your lawyers in bringing that claim will be wasted.
If the claim is not struck out but runs to trial and you win, your judgment award may be severely reduced by an order that you pay other side’s costs up to and including the trial (especially if the mediation had a good prospect of resulting in a settlement). I strongly urge you to attend your mediation. There is nothing to lose.