Mediation settlements are voluntary. No one can force you to agree.
Just because you have agreed to mediate does not mean that you need to agree with the proposed settlement. If you do not agree, then there will be no settlement agreement. Make the most of your time with the mediator; think carefully about why you do not want to accept the offer. Is there any way in which it could be amended that would make it more acceptable to you?
If you or the other side has commenced legal proceedings then it is likely that you will consider restarting the process. After the mediation your options are still open and you can consider how you want to proceed.