Hi Emma, sorry to hear you’re having problem with your meringue sinking. There are several reasons why this could be happening:
It’s important to use white granulated sweetener that has the same weight and texture as sugar rather than a powdered type.
The egg whites could be over or under whisked; they only need to be whisked for 1 – 2 minutes on high speed, until stiff peaks are formed.
Try using egg whites at room temperature; they can hold more air than cold ones.
Add sugar and sweetener a little at a time; adding it all at once can knock out the air you’ve incorporated.The meringue should be stiff and glossy after the sugar and sweetener has been incorporated.
Make sure your bowl and whisk heads are completely clean and dry.
Make sure you haven’t got a speck of yolk or shell in the egg whites.
Check your oven temperature is accurate, it maybe running a little cool.
Don’t open the oven door during baking.
If you made these on a humid day, it could be that the meringues have absorbed moisture from the air and caused them to collapse.
You could try whisking in 1/2 tsp lemon juice once the sugar and sweetener has been whisked into the meringue; this can help stabilise meringue.
We do hope these suggestions help and you have success next time!