Our Gemara on Amud Beis quotes a verse from Mishle (14:10), “The heart knows the soul’s bitterness” as a support for the idea that even when the expert physicians do not consider a condition life-threatening, if the person himself does, we treat it as valid.
The simple meaning from the Gemara is that we should not judge another person’s pain. Many experiences of mental illness such as trauma, depression, anxiety and the like are highly personal subjective experiences. What one person finds unbearable, might for Another person seem trivial. One of the most damaging things to say is, “Can’t you just get over it ?”.
We must take our cues for our Sages, who never trivialized physical pain, and even emotional pain. One of the strongest examples comes from an odd case of love sickness described in Sanhedrin (75a) where a man became so lovesick, that the doctors attested to the rabbis that he would die if he could not fulfill the object of his desire. The rabbis spent much time deliberating over how or if his need could be met. The rabbis, with great compassion and no cynicism at all, took the doctors’ description of his emotional subjective state at face value. They did not declare, “it’s all in his head”. They also didn’t tell him to study more mussar and forget about his fantasy. A person who is suffering is never to be told, “You should just move on”, “suck it up”, “it’s all in your head” and the like.
But, what do you do If you believe that someone you love is stuck and is obsessing and ruminating about matters, that by all rights, they should let go off? First of all, give them a lot of time and compassion. Second, if it continues to persist beyond a reasonable timeframe and is significantly interfering with enjoying life, you could respectfully say, “I am worried that somehow you are stuck in his feelings. I know it is difficult for me to judge, and I’m not criticizing you for it nor can I tell you that I know the truth. But I am wondering if you would consider that somehow you are more stuck than you realize.”
If the person remains stuck, you could try it again a few weeks later. In the same respectful way, you can ask if they are willing to get help from a third-party who could be objective. Ultimately, it takes a lot of patience and different people experience pain, trauma, and depression very differently. Some things cannot be rushed even if you’re in a hurry.
Translations Courtesy of Sefaria, except when, sometimes, I disagree with the translation 
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