Oh wow!!!!! First off...way to go. Good for you trying for that long, many people would have caved before you. (you are not "caving" just making the smart choice) best for you and the little guy! not cool at all. Do the parents know about the crying the whole time or did you sugar coat alot? Hopefully they know and it wont be hard at all to inform them of your choice and they should completely understand. I dont do the computer communication like most you guys, everything I do is person to person, but same thing really... I would just be really respectful toward them and make sure I started out by telling them how much I appreciated the opportunity to take care of their child and how sweet of a boy he was. But just inform them that after a lengthy "trial period" you have come to a decision that this was not the "right fit" for either of you. Gear it toward their childs needs, not your stress or inability to deal with the crying. just tell them you feel for his sake termination of care is the best choice for their child in your opinion. wish them the best of luck and leave it at that.
Good Luck!