DD was fed to sleep until she was 9months, so a different age - but I found the thing that worked was, as RoyaUk said, a change in routine - we started doing the bedtime routine differently, it took a week or so, but in the end, we got there and without tears. having said that, the routine has changed a number of times since! We also introduced a teddy for her - she had never shown much interest in one, but I started to put her to sleep with the same teddy and soon it became a sleep association.
And I also second the Elizabeth pantly No cry sleep solution - I may be wrong, but I think she might have released one aimed at toddlers Another thing we introduced was key words which Elizabeth Pantly talks about, which become another sleep association. To start with, you say the words as they are almost asleep, so that they associate them with sleep, and then you'd start saying them earlier in the settling process. Our key words were night, night, sleepy time, see you in the morning.
Good luck, I hope things improve!