The difficulty is actually very gradual and rewarding. :)
Yes it's a hard game but only depending upon how you opt to play the game. Otherwise, it's a stellar single-player experience. ^^
Other single-player games I can recommend (some Indie darlings, some known titles): The Binding of Isaac, The Legend of Zelda, Crash Bandicoot, Maneater, The Stanley Parable (Ultra Deluxe if possible), Dead Cells (though that is a REALLY difficult game), Slay the Spire, Little Nightmares (second one is also good), Limbo, Thomas was Alone, Portal, Bioshock, System Shock 2, DOOM and lastly I'd say Sundered (hopefully the Eldritch edition).
EDIT: I forgot about Hades, Grim Dawn and Diablo II. xD