I’m almost afraid to respond because I’m NOT that mechanically minded, but have you at least checked the oil level? If oil level is low, the shifting action gets messed up. Fill up to the correct level and see what happens.
Haven’t changed oil in 7000 miles? Are you at least using full synthetic oil? But as long as the oil is still oil and not all turned into thick gunk, I don’t think the shifting problem is from how old the oil is.