I experience erratic hard shifting on my 2004 Buick Century. It has around 120,000 miles with no prior repairs on the transmission. No leakage. The hard shifting seems to occur after the car idles for a long time, especially when I’m stuck in bumper to bumper traffic.
Not the best transmission… but, have you had the fluid changed, ever? Try dropping the pan and take a look. If the transmission needs replacing, it will be obvious. If it look decent in there, install a new filter and fluid and drive it a week. It may improve things and maybe you’ll get another 30 to 50K miles before it needs rebuilding.