Split Nights SOS

[SplitNightsHell]

Admittedly dramatic title but please help!

My 13 month old has been having split nights since she turned 9 months. It's nearly every night and I'm exhausted. Because it's been so long, I don't even really know how much sleep she needs in a day. I've tried many times to resolve it but the split nights kept coming back.

2 weeks ago I calculated that she had been having about 12 hours sleep in 24 hours so I tried to set our day up to look something like this

7am - wake
2 naps totalling 2 hours, finishing by 5pm
9pm - bedtime

This would work for a few days but she would get more and more tired, her night sleep would get increasingly disrupted including split nights and very restless sleep. She would need a mammoth catch up day where she would sleep 13.5 hours or so. She doesn't need that amount of sleep everyday so that's not the answer but she clearly gets overtired. I just don't know what to do. I thought after 2 weeks of 7am wake ups her rhythm would have adjusted and her sleep would have consolidated but if anything it's worse. She went from 12 hours per day one week to 11 hours per day the next week.

She is a generally good sleeper if the balance is right. Even during the last few months, we have had her sleeping through the night occasionally, but copy and paste of those days doesn't work for the next days.

Has anyone dealt with similar and overcome it?

How long would you say her wake windows are at present? Is she going down easily for her naps and at night?

Is she awake long enough after her last nap? Some children need the last wake window to be a bit longer.

Could she be in the awkward 2 naps to 1 nap transition? Going from ~4 hour wake windows (2 naps, 12 hours of sleep total) to ~6 hour wake windows is quite a leap.

@Mummyandmini we've considered it and I contacted one about a month ago but she wouldn't have had availability until April. I thought we'd definitely have sussed it by then 😭 We don't want to sleep train but I'm still wondering whether a sleep consultant might help us find the balance between night and day that we're clearly missing. Just very expensive for a routine that she might outgrow in a month or two!

@skkyelark About 3 hours before her first nap, 4 hours before her second nap and 4.25 before bedtime. She's often asleep within 10 minutes but sometimes takes up to 15. We follow her cues but I read up on possums approach so we tried that and saw her wake windows lengthen but I think it stretched too much and was unsustainable for her. If she has a long first nap she does fight the second nap so we tend to cap the first one and she goes for the second one about 2.30.
Occasionally she has a 1 nap day but normally only if she's had a lie in, and in that case she might go 3.5 hours before the nap and 6.5 after it - this is what happened today. Less awake time today so we'll see what tonight brings but she was shattered. She can't do successive 1 nap days so I don't think she's ready to drop a nap yet.

So my now 3.5 yo had awful awful split nights from 9 months for (I hate to say) well over a year. I tried everything. Changing naps, dropping naps, wake windows, tried a sleep consultant and nothing worked we simply had to ride it out. It was hell working full time I was so tired.

waking up at 7am and going to bed at 9pm sounds like a long day though. What time generally are her naps and for how long?