SSI caps very low. It's not meant to cover every one of your expenses. It stands for SUPPLEMENTAL Security Income. If you are unable to work I would suggest also looking into dietary benefit programs (food stamps) and housing assistance.

I'm surprised that at your age you were awarded benefits for a foot injury. You must have really fucked up your hoof to get benefits at the initial level. Hope your healing process goes well because living on SSI is a rough way to get by. I'd explore training for lighter duty / office jobs. Your MMA career was probably a dicey prospect anyhow, but luckily you're young enough that you've still got a lot of time and potential.