Advice needed on sleep training 18 month old

[Hydrangea60]

I've posted about this before but I'm completely unable to cope now so posting here for traffic.

My 18 month old was a fantastic sleeper up until he got his canines 2 months ago. Since then we've had issues with bedtime battles, early wakes and split nights. The first two seem to have improved a bit but the split nights are still ongoing. He'll wake at 1 and won't go back to sleep until 3-4.

We've tried adjusting nap times, bed times etc and it hasn't worked. Co sleeping doesn't work as he just messes around and seems to see it as playtime. I think we need some sort of formal method but I feel completely overwhelmed with information and don't know where to start. I just don't understand how he could self settle at 3 months old but has now forgotten how to do it.

I feel awful about sleep training and letting him cry but can't think of what else to do. It's affecting my work, relationship, my physical and mental health. He's high sleep needs as well so on the really bad nights the tantrums are awful the next day as he's so overtired.

Any help would be great.

As an early childhood specialist, I don’t recommend sleep training ever ever ever. Can you share his current routine for a better idea?

This is what I'm worried about but I feel I'm a terrible mum to him at the moment as I'm so worn out by the sleep deprivation 😞 we're all miserable and I can't see another way forward. This obviously isn't a normal regression as it's been going on for two months now. He never had any issues or regressions before this so it's all completely new to us.

His routine is (roughly):

Wake up - 7-7.30
Breakfast - 8-8.30
Lunch - 11-11.30
Nap - 12 for 1.5 to 2 hours
Snack - 3.30-4
Dinner - 6.30
Bath - 7.15
Bottle and book - 7.45
Bed - 8

I don't think this is an unusually high amount of sleep for his age, particularly as he's always been high sleep needs. We can't cap the nap as he gets so grumpy in the afternoons and evenings when we've tried. He also goes to nursery 3 days a week and they have a policy of not capping naps.

Any suggestions would be great.