If you are worried about the bending of the simply supported beam than you could make it a truss (as Brad and DKR suggested). I guess this depends on the roof and wall design and how far into the build you are. The king post and queen post in their respective trusses work in tension thereby negating some of the bending stress you are worried about. It all depends on what is done and what you have designed already. If you have a sufficiently supported ridge a cable might even work anchoring the center of your beam. If you don't want to support the load from the bottom with a post, the other option is to pull it up. Im not an engineer, just some thoughts. Good luck with your project.