DIAGNOSIS That loose, floppy feeling means the little spring or detent ball inside the forward/reverse mechanism has likely broken or worn out. It's a common issue with heavy use.
FIX You can try a temporary fix first. With the drill off, spray a tiny amount of electrical contact cleaner (not lubricant) into the sides of the lever mechanism while moving it back and forth. This might wash out debris that's jamming the spring. Let it dry completely before using.
IF THAT DIDN'T WORK The permanent fix is replacing the entire forward/reverse switch assembly. It's a single module that includes the lever and the internal electrical contacts. You have to open the drill's housing to get to it. It's a straightforward swap if you're comfortable with taking the drill apart.
HEADS UP A lever that slips to the middle (lock position) while you're driving a screw can stall the drill and strain the motor. It's annoying, but it won't damage the drill immediately. Just be aware and don't rely on the lever staying put until you fix it.