First see if the spring is weak or set wrong. Do this by disconnecting the trolley and see how difficult it is to raise the door by hand. Raise the door to about 5 feet up from the garage floor and release it. Does the door stay in position or does it fall to the floor? If it falls, then the spring should be wound tighter until the door stays in position. Winding the spring tighter is a professional job unless you are experienced in how to do this safely.
EDIT You have an old door and an old opener. You report that you have no sensors on the doors. I assume you mean the optical sensor which reverses the door if the beam is interrupted. This has been standard on residential openers for decades (not generally required for openers in industrial use), but getting a new opener with these sensors could prevent damage to equipment or injury to a pet or to a person, especially to a child.
It is almost certainly time to call a competent professional service to evaluate the the condition of the door and opener and perform maintenance.