In DH, when the trio are in the vault, Hermione uses levacorpus to hoist
Harry up to get the cup.
HARRY UN-jinxes HIMSELF with Libracorpus. I don't recall any other instance in the books where anyone who was jinxed - jelly legs, whatever, was able to un-jinx themselves. Maybe that was
Snape's big discovery. A counter spell you could use on yourself?