The STX series had the steering built into the lower dash section below the fuel tank.
If yours is green, it is the original style.
The black lower sections were beefed up and usually last longer.
Most likely that section broke and now your steering is all wacky.
I have replaced alot of these over the years.
Order a new lower dash section or find a good used one.
If buying a used one, make sure you buy a black one!
To do the job, it's pretty straight forward swapping parts over to the new section.
Here are the pics of the parts diagram assuming you have a yellow mower deck.
If you have a black mower deck, let me know and I will attach those as well.
**The key to making the steering last longer on this series(or any series) is to regularly grease the front axle spindles and keep your front tire air pressure at about 14psi. But sometimes, parts just break no matter what you do. I have 2 of these machines personally and have never broken the steering...yet, but have replaced many here at the dealership over the years.
Will