Reduce stem growth of flowers

I thnk the basic problem is that they are much too crowded together, so they are growing to try to get more light.

Even small flowering plants need to be 8 or 12 inches apart to develop normally to their full size. If you sowed the seeds straight into the container, you need to sow more seeds than that since they won't all germinate, and then thin out the seedlings when they are big enough to see which are the strongest plants.

This is a common beginner's mistake. Tiny plants only an inch across that are spaced a foot apart might look a bit silly until they grow bigger, but that is what will produce the best results in the end.