you pruned it back way too far. New growth is not strong growth, of course it'd droop.

Cut it back to the point where it'll stop drooping, probably down to 12-24 inches, and let it grow from there. Then in the future never cut it back so much.

forsythia is a plant that you can easily prune to keep in check like any other hedge shrug. You can even prune it into shapes. But cutting it back to just above the ground will NOT help it grow bushy.