Over-brushing is unfortunately a very common problem, and can be surprisingly destructive. There are two problems we see regularly caused by over-enthusiastic brushing; gum recession and V-shaped notches worn into the necks of the teeth (root abrasions). Both can be caused by scrubbing hard, especially with a hard bristled brush. The brusher is usually either in a hurry, or is eagerly trying to get their teeth extra clean unaware of the damage they are causing. If you scrub the front surfaces of your teeth hard and fast you are probably over-brushing. Having a toothbrush with splayed bristles is often a sign.
This picture shows how gum recession can be caused by brushing too hard. It often affects the top side teeth more severely as this is the most comfortable position for hard scrubbing. Recession can sometimes be treated with specialist regenerative gum surgery but is often difficult and results are variable. It is best to try and avoid the gums receding in the first place.
These V-shaped notches in the root result when the gum has receded exposing the root dentine. The root dentine isn't as hard as the enamel and with repeated hard scrubbing it will wear away. This can make teeth sensitive and weaken them. These V-shaped notches can be repaired with tooth-coloured fillings.
The best way is to use an electric toothbrush. It is hard to brush too hard with an electric brush. However it is still possible and there are some electric brushes that have a warning light or beep if you are brushing too hard. When using an electric brush don't try and brush as you would with a normal brush. Just hold the brush tucking the bristles into the gum line and slowly move back and forward over a couple of teeth for a few seconds, before moving to the next teeth along. If using a manual brush hold the toothbrush to the bristles are tucking into the gumline. Brush backwards and forwards a few times, not too hard, then flick and drag the brush down the tooth surface pulling the plaque away from the gumline.