Hi there. I just looked at your previous posts to see how old your LO is, and saw you had an unanswered thread on this problem. I'm so sorry you didn't get any replies. We do try to make sure all threads are responded to by someone in the community but obviously some times things are overlooked.
Sounds like you are really struggling there but you have done so so well to remain calm and keep helping her, well done Mummy! I saw in your previous thread you had thoughts creeping in to just leave her to it, I'm sure if you hold tight you will get through this difficult phase without breaking your wonderful bond with your DD.
At 15 months she does need more than a 30 min nap. It is not 100% clear how long a nap she needs, here you say she was doing 2-3 hrs and in your other thread you said 1.5-2hrs...it is helpful to know how much a LO was in the habit of when we start looking at a routine tweak, so lets go with 1.5-2hrs (rather than 3hrs), OK?
Seems she needed a longer activity time before her nap, that's what I'd gather from the short naps and WU time moving earlier. So I would go forwards by moving her set nap time later. Move in 15 mins every 2 days until the nap extends. When this happens also move BT 15 mins later (I saw on your other thread you'd be happy to move BT later to encourage a later WU). Her set nap time might need to move considerably, the first aspect is that she needs a longer A time but the second aspect is that if WU moves later then nap also needs to move later, so keep this in mind otherwise it can seem like a huge shift.
I'd think you might be looking at something like:
WU 7
S 1.30/2 - 3.30
BT 8pm
(move in 15 min steps not all in one go)
In the first days moving her nap time later might mean she is OT but considering she is tired and grumpy all afternoon already and only taking short naps I don't think it's really going to be much worse than it already is, yk? When she wakes resettle in her cot, PU for a cuddle if needed but I'd put her back into her bed for the rest of the nap.
You could also try a W2S just before the 30 mins. Sometimes it is easier to resettle before they fully wake and cry out, so I'd nip in around 20-25mins into the nap and either begin to sooth or be right there ready to help settle and the first hint of waking, a firm hand on her back to rock her slightly (like the pram) or stroking her head, whatever you think is helpful. W2S isn't something you continue for ever but is a useful tool during these tricky times. There's a chance it will reduce the resettle time which will likely feel less stressful for you.
What do you think?