When used as a genuine problem-solving activity worrying is a positive process that helps us to anticipate and deal with life's threats and challenges, and that's why most people will say that they worry at some time in their lives. However, some forms of worry can become maladaptive, and instead of thinking through and solving problems, the pathological worrier will become locked into a catastrophizing style of thinking that merely tries to identify potential threats and challenges instead of trying to resolve them. This is the common "what if...?" style of thinking which merely exacerbates stress and anxiety rather than relieving it.

There are a number of factors that contribute to making worry seem like it doesn't have an "off" switch. One important factor is the beliefs that chronic worriers have developed about worrying. Most chronic worriers have established implicit beliefs that worrying is an important thing to do, and if they don't worry then bad things will happen. This makes worrying almost a reflexive response to any event that may seem threatening or challenging. Studies have shown that over 90% of what pathological worriers worry about never actually happens, and worriers often believe their worrying had something to do with preventing the bad things happening (even when it didn't). Some therapies, such as metacognitive therapy, try to alleviate pathological worrying by helping the worrier to identify and change these ingrained beliefs about the need to worry. For an overview of these types of therapy see https://journals.sagepub.com/doi/10.5127/jep.007910.

In my view there's no such thing as a "born worrier" - there's very little evidence for a genetic cause to worrying, and chronic worriers tend to think they were born as worriers mainly because their worrying has become uncontrollable so they assume they must have been born that way. There are quite a few developmental factors that we now know are risk factors for pathological worrying. These include parenting that leads to insecure attachment styles, experiencing unexpected negative life events during childhood, and a reduced capacity to regulate emotions.

If you're a chronic worrier, your particular form of worrisome thinking will have been developed over many years, so there is no "simple" way of dealing with it because for chronic worriers worrying has pretty much become an uncontrollable obsession. One common way of beginning to manage worrying is by confining your worrying to a well-defined time period during the day. You can worry as much as you like during this scheduled "worry time", but for the rest of the day you should just note down your worries and go back to them only during your "worry time". This helps to prevent worrying from becoming a 24 hour daily activity that can blight the rest of your daily life. You might find this worksheet published by psychologists at the University of Exeter helpful http://cedar.exeter.ac.uk/media/universityofexeter/schoolofpsychology/cedar/documents/liiapt/Managing_Your_Worries.pdf

Almost everyone will have some "real" things to worry about during their lifetime, so it's important to use worrying constructively rather than obstructively. Agonising over a problem and merely catastrophising it is obviously not helpful (the notorious "What if...?" questioning style). This process simply causes greater levels of stress and anxiety, and leads the worrier to doubt themselves and their ability to deal with the problem. In order to deal with real life problems, most forms of therapy for pathological worrying include a problem-solving skills component, which provides the worrier with some skills required to manage and even solve the worry problem. This may involve learning a systematic way of dealing with problems and defining solutions. For an example, see http://web4health.info/it/psy-problem-solving.htm

'Worry' tends to be used to describe anxious thinking about future events (such as future threats and challenges) whereas rumination is more often used to describe repetitive thinking about either past events or depressing events (such as rumination about feelings related to past losses and failures). But as cognitive processes they're very similar, and many of the factors that make worrying uncontrollable also make rumination uncontrollable (for example, see https://www.sciencedirect.com/science/article/abs/pii/S0272735811001371)

There is plenty of evidence that pathological worriers do appear to have an attentional bias for negative or threatening information. This is a pre-conscious bias that grabs attention and initiates worrying. But an attentional bias is only one of many factors that contribute to chronic perseverative worrying, so while attempting to eliminate a negative attentional bias is helpful, in my opinion there are also a number of other factors unrelated to attention that will need to be addressed to help an individual manage their worrying. There has been a good deal of research on the effectiveness of cognitive bias modification (CBM) techniques, and I think it's fair to say that the jury is still out on how effective they are in helping to manage symptoms of anxiety and worrying - especially over the longer term.

Chronic worrying is a stressful activity. If you are a chronic worrier then the simple act of worrying will make you more stressed and more anxious, hence mindfulness is seen as a helpful way of trying to manage this stress. But more than that, stress, anxiety and negative mood are not just consequences of worrying, they are actually direct causes of worrying. Negative mood will make you worry for longer, it will make you think your worrying hasn't been helpful and so you should continue to worry, and it will also make your worrisome thinking negative and obstructive rather than positive and helpful So any activity that can lift a negative mood should have beneficial effects on worrying.