Check out the link below and scan down to the maps and check out by clicking on the map of those species that are shaded for Florida-really quite a few.

Here is a link that might be useful: Native Hypericums of USA

Note that not all of the plants on the link to USDA are native, the blue shading indicates native. For example, Hypericum perforatum is not native (but luckily I don't think that is what you have based on the flower).

Happily I stumbled across this plant while looking for other opposite leaf yellow flowering plants. I think it may be your plant!

Here is a link that might be useful: Ascyrum hypericoides

Carol23, thanks for the link. That looks very much like my plant. My plant is 3 to 4 feet tall, and Ascyrum hypercoides gets to be about 18 inches tall. However, it might get taller in North Florida.

Also, thanks to terrestial_man for the link to the Hypericum page at USDA, and the tip from esh_ga.

There is another 4-petaled Hypericum species, along with St. Andrew's Cross, that is native to FL.
You will probably need to examine plant habit, stature, foliage and bark, to determine which one you have.

Hypericum tetrapetalum
Rb

Perhaps it behaves differently in FL, but Hypericum hypericoides grows 3-3.5 feet tall here in Tennessee.