You have to continue writing the checks to the mother unfortunitally, although, now that your daughter is 18, you can go to the courts and file an amendment to the original court order and request that payments be made payable to your daughter instead from now on.
A girlfriend in college was able to receive her fathers child support checks while she was in college housing because she was not living with her mother at the time. Since she was 18, she was allowed to use the checks for her school directly instead of her mom stealing them again.