Refusing to sleep

My little boy is 21 months old.  We have a routine of dinner, bath, story and milk and then bed at 7.30pm. He has never settled very well in his cot as we co sleeped. I tried controlled crying just before Christmas (which was extremely difficult) which worked for about three days and he slowly regressd back. He now will not even entertain his cot and screams the house down everytime i try to put him in it. He has started to make himself sick when i do put him in it. Im on my last legs with it all. Any advice or reassurance would be much appreciated.