How to get a previously good at settling just turned three year old to settle in their bed alone

[pinklemonadeandchocolatecake]

...as the title says! From about four weeks ago she has lost the ability to settle herself to sleep in the evening, she needs me to sit and hold her hand . She is still in a cot as hasn’t attempted climbing out so there has been no need to change as she was happy. Gradual retreat didn’t work, do you think it would be ok to put her to bed and say I will be back in two minutes, she will cry and then settle when I go back, then go in after three minutes etc? I didn’t mind staying with her but it’s gone from sitting and her falling asleep in ten minutes to 40 minutes the last two nights. Any advice is welcomed! Thank you.