Check your door springs. It sure sounds like you may have a broken one. Also remove the cover and check the drive gear. It is a white nylon gear just in front of the motor and may be bad. It should be obvious if it is.

I had the same problem. Our handyman fixed it for us - it was the spring, somehow came loose or something like that. Nothing wrong with the opener itself. Also, the laser thingies on the bottom inside sides of the doors were off level - those are the little safety lasers that automatically open the door back up when something (or someone) is in the way of the door closing.