After a few dry weeks the road holds everything cars and bikes have left behind: dust, oil, rubber. The first drops turn it into a greasy film. The first quarter of an hour of rain is therefore the trickiest.
It helps me to take one deep breath at that moment and leave more distance. Out of caution, because the road is different now from ten minutes ago.
How to brake properly on a wet road is best practised on a recognised rider safety course. The head is yours, and it is allowed to stay calm.