That is the nature of all flowers in the rain especially medium tall or tall flowers. The water fills the spaces in the bloom and makes it too heavy for the stems to support. It is why peony flowers fall apart in the rain and Delphiniums fall over.

Cutting them back is not the answer. You'd be cutting off the blooms. You can try giving them support by criss crossing short pieces of bamboo in the pot or somehow staking them. You can also go out after a rain and give them a gentle shake to remove the water from the blooms.

Next time choose a more compact plant for your planters

I have mine staked in clumps. I put one green bamboo stake in with green twine looping around the clump keeping them generally upright. Every once in a while I need to pull the twine up as it tends to slip lower (or the plants get taller). Seems to work reasonably well for me.

Lisa