As some people said, the support of professional specialists is really foundamental. Be aware that with a psychologist you (in this case your partner, I am speaking at first person) need to empathize and this means that you should change specialist if you don't feel at ease or you don't feel suggestions good for you. Not every psychologist is for every one and also there are differents schools of psychology with different approaches, you have to found the good fit (my suggestion is cognitive behavioral school)
As brudgers said you can't do the work for him/her, you can help for sure, but this can be difficult and being too protective is not a solution. At one point you should have faith in your partner being able to come out. But something has to trigger inside your partner... you can't do the job for him/her.
Be aware of methods/techniques that claims to resolve it easily. Go with a psychologist, maybe you/your partner are smart enough to understand the cause of the problem, but you don't know right techniques and how to apply them properly.
In general Sport helps, there was a study by an Italian Neuropsychologist that showed how 30 minutes of aerobic sport was like a prozac (I'm simplifying, but the concept is this).
Do things, action instead of thinking, this helps.
Those errors are the ones that you catch by experience. That means making a comparison with previous done (and well finished) jobs. a When I refer only at theory I feel unsafe for the final result, so I try to search for similar jobs or asks experienced workers. I am sure that experienced workers also in this case of the submarine were complaining that was too heavy. Experienced workers base their "calculations" work on previous done jobs and this is really good to give an idea about if is going to work.
In general Sport helps, there was a study by an Italian Neuropsychologist that showed how 30 minutes of aerobic sport was like a prozac (I'm simplifying, but the concept is this). Do things, action instead of thinking, this helps.