Do you think Littlefinger ever realized Catelyn didn’t love him?

No. I think he still believes:

“You are old enough to know that your mother and I were more than friends. There was a time when Cat was all I wanted in this world. I dared to dream of the life we might make and the children she would give me … but she was a daughter of Riverrun, and Hoster Tully. Family, Duty, Honor, Sansa. Family, Duty, Honor meant I could never have her hand. But she gave me something finer, a gift a woman can give but once. How could I turn my back upon her daughter? In a better world, you might have been mine, not Eddard Stark’s.”