You stop worrying by learning to not pay attention to those thoughts that lead to worry. Training your mind to pay attention to the reality of the moment achieves this.
The clarity of mind and focus that results enables you appropriately deal with the issues that trigger the thoughts that lead to worry. This contrasts sharply with how a worried mind often makes the issue at hand worse.
The difference is between the mindfulness of being focused - so that you do just what you need to do to achieve your goals or objectives - and the mindlessness of worry that actually pours your mental energy into what you don’t want to happen!
Daily mental exercising enables the mind focus on reality instead of “thought reality”. And, when you become focused, everything becomes easier.
In short, developing applied mindfulness enables you put an end to worry and all its side-effects like anxiety, fear and panic.