My 18 month old has always been sporadic with his sleep, and went through a not great phase at about 6-12 months, where he needed milk a lot to settle through the night. From 12 months to about the last month or so he's been good - he would go to bed at 7ish, wake grizzling or crying within a couple of hours virtually every night, but only need a quick cuddle to go back to sleep.
He's gone downhill a lot in the last few weeks and we've completely lost our evenings and he's also waking in the night so we're really tired.
He goes to sleep on his own fine, with no issues. He's a head banger and moans a bit when he does this but always goes off to sleep on his own (after a bath/bottle/story bedtime routine). But then after anything from 30 minutes he's up, crying, moaning, head banging but working himself up to the point he's really crying, and we have to go in. This ends up being constant all evening, DH and I take it in turns, cuddles, milk if needed, nothing specific seems to work. Sometimes he will finally settle around 10pm and sleep until 6:30 ish, sometimes he's up more than once in that time. He doesn't have a dummy, he has a comforter which is always with him.
We are constantly trying to get everything right eg what we dress him in, window open/closed, fan on/off. He has a Ewan sheep too so white noise.
He cut his 16th tooth a couple of weeks ago so I don't believe he's even teething. In fact his sleep was much better even when he was cutting teeth.
He naps in the day, around 1-1.5 hours usually. He gets plenty of exercise.
We tried controlled crying a couple of weeks ago as we just felt so defeated, but he worked himself up so much that he was sick when we'd only reached I think 10 minutes of waiting before going in for a cuddle (we started with 5 minutes then planned to increase by 5 minute intervals).