The motor should not be that difficult to turn. I would loosen the worm gear and bushings just a bit and check but they need to be fairly tight. Maybe the bearings are going bad.

Hey home owners. Hire a Pro. We make it sound easy because we've done it a hundred times and we have all the right tools. There are a lot a variables that cause the gears to wear out. If you hire a pro they can fix the opener and make sure everything else is working properly so you get the longest life out of your new gear. Homeowners are always having problems with the openers after they try to fix them because the main gear drive may just be a symtom of a problem and not the main problem. A lot of times a mom and pop type door guy is your best bet because they deal with service more, not just new construction so they see these problems every day. Know you limits when it comes to DIY.