Jerry Sandusky Letters from Prison -- 'I'm Trying to Get Better'

Convicted child rapist Jerry Sandusky says he's having a "very challenging" time behind bars but insists he's "trying to get better" ... this according to prison letters obtained by TMZ.

Just like Aaron Hernandez, Sandusky has been communicating with multiple pen pals he's made from inside his Pennsylvania prison cell over the past 9 months ... and it's clear, the 69-year-old is struggling.

"I've been quite confined, always searching for purpose," Sandusky writes ... "It is very challenging."

In the letters, -- one sent in May, another in June -- Sandusky is still playing the victim ... calling his conviction a grave injustice. He vows to continue battling the case "until the last whistle."

Sandusky also touched on his "personal battles" -- and while there's no specific reference to pedophilia -- Jerry ensures his pen pal, "I'm trying to get better."