Since you two have a handicapped child together, I would suppose that the best thing is to try and stay together....I have a question for you...why would you do something so cruel and vile as to sleep with his best friend. I mean of all the people you could have picked to have sex with, you picked one your BF's best friend. That is just cruel. Do you harbor some hatred towards your fiance? Your choice of affair partner is not a mistake.....this was done in part to harm your fiance...the question is WHY?