If you do have an accident - and I will be very, very upset with you if you do - you have to ruin it. Stop stroking completely, jam your thumb over that cockhead, make every effort not to spasm any more than you absolutely have to. It'll hurt. It won't be satisfying or enjoyable. And you certainly won't be happy that you did it.
And that will be one of the only twelve orgasms you have this year. All because you couldn't hold it in when I told you to!