I would advise using a strong clay like Cernit,
But also, mix in some of their translucent to the clay to add even more strength. Bake at the max temp for the clay, 130c for at least an hour, taking care to not allow the clay to scorch or overheat.
That way you will get the clay to be as strong as it can be - but you know, even then I still remain unconvinced that polymer clay is really well suited to very fine and delicate things.
The key is to make these types of petals and flowers in such a way that they look delicate but still are actually quite sturdy enough in all the right places and that takes trial and error, practise etc