Poor you, my ds has always been a poor sleeper - he is 25 months now and at the moment going through a spell of sleeping until 4 and then goes back easy until 6 which is close to as good as it gets for us.
i tried everything - read all the books and spent alot of time on various techniques but in the end only one thing worked for me which i read in a book which is called 'Getting your baby or toddler to sleep' which basically says that at a point where they are well, no major changes (at nursery or at home), not just back or just going on holiday, you need to put them to bed, tuck in say goodnight and tell them you will see them in the morning, then leave and don't go back in.
Basically the first time we did this it took 4 nights for him to stop waking and sleep through, the first two nights were awful as he would cry for half an hour at bedtime, then wake up 3 or 4 times and cry for twenty, thirty, forty minutes - which is a long time if you are tired and lying awake in the night - i used ear plugs to take the edge off and dh slept downstairs because he couldn't handle the crying, but by the third night the xcrying and waking were less and by the fourth night he barely fussed.
now we take a totally routine based approach to bedtime, as soon as he comes out of routine he starts night waking - changed class at nursery = nightwaking, holiday disruption =- night waking, but now I use this technique whenever we get home and it only takes one or two nights to get him sleeping better again
may sound like a harsh approach and I wouldn't do it with a child that was under a year and didn't understand, but by 21 months your dc is old enough for you to explain and for them to be able to learn that once you say goodnight you will not go back
tried controlled crying but that just seemed to teach him that if he cried long enough i would go back, tried gradual withdrawl but that was too frustraing and difficult once i was back at work to spend an hour creeping out of the room 3 times a night, tried music, white noise, blanket, cuddly toy, - basically this has been the most effective way
bad sleeper is the worst and sleep deprivation so awful - bestof luck!