Hi.
My 15 month old did this until very recently. I got so exhausted by the fact that in over a year I hadn't had a night's sleep that I resorted to just getting him into bed with me and giving him the breast whenever he wanted it. It ended up with him getting in my bed from about midnight and feeding every hour or two. It was hell
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The way I have dealt with it (touch wood) is by withdrawing each of the elements of the sleeping pattern one by one.
I first of all withdrew the breast at night, but still breastfed morning and evening (I gave him a bottle before his breastfeed at night so I wouldn't worry that he was hungry). So he would come in my bed at midnight-ish but would not get a feed until 7am. He was not at all happy with it and screaed in my face most of the night, but at least he got some comfort and familiarity. When it was time for morning feed at 7am I made sure the light and radio were on so he could not mistake it for a night feed.
Then after a few days of this (by which time he had stopped screaming the house down and got used to it) I started to put him in his cot awake so he would get himself off to sleep. Again this was not very popular, especially as he just refused to lie down and on occasion I have to admit I held him down with both hands until he fell asleep. I still got him into my bed when he woke up.
Then after a few of nights of that (when he learnt to lie down and get himself off with only a bit of patting) I stopped getting him out of the cot when he woke, and did lying down and patting in the cot until he got back off to sleep. He didn't mind this much by this time.
We've been on the last phase for about a week and now he is sleeping through until anytime between 4.30 and 6.30 and goes back to sleep in the cot within a few minutes with a bit of patting when he does wake up.
The only thing left now is for me to move him into his own room next door, which has been ready for months! I just feel I will miss him, but I guess all good things have to come to an end....
Good luck!