These are the Rebbe's letters to individuals about their own situations. Where worry or low mood was a medical matter, he told people to follow the advice of an expert doctor alongside everything below (IK 5790, 8804). See also: Health – Anxiety, depression, nerves and sleep.
Trust takes away the ground for worry. Contemplate deeply that G-d guides every detail at every moment, and the foundation for worry falls away; we are to act naturally and leave the rest to Him (IK 10962). Trust in His individual Providence dispels fear (IK 1442, 5757, 6247), and brings true security and peace of mind (IK 907, 929). Strengthening trust in itself brings the blessing sooner (IK 1615, 2580, 10918). Unfounded worry only drains one's strength — put that energy into positive action instead (IK 819, 1284).
Don't fight the thoughts — change the subject. Battling worrying or troubling thoughts directly only strengthens them. Turn the mind to Torah study, mitzvos and helping others, and the thoughts weaken on their own (IK 4548, 4956, 5170, 5400, 5790, 6600, 6847). Fill your mind with Torah, especially Tanya and Mishnayos learned by heart, and there is no room left for them (IK 1374, 1457, 1642, 1724). For wandering thoughts during prayer or study, keep a siddur or sefer open and look at the words (IK 1153, 4663, 8184). Obsessive worries about whether a mitzvah was done right are best ignored, not argued with (IK 10951); for worry about past mistakes, increase tzedakah (IK 3005).
Fear. Fear comes from the yetzer hara; answer it with trust in G-d's constant care, Torah and positive activity (IK 2656, 10950). Don't be driven by baseless fears (IK 4932), or by fear of what others might do to you (IK 10508). A frightening dream is no cause for alarm (IK 4666).
Worry about health. Follow the doctor's advice, but don't dwell on your condition or read every report — anxiety about health often comes from imagination (IK 1362, 1633, 2903, 3248, 4031). Keep calm about a loved one's health and let the mind guide the emotions (IK 4136).
Worry about livelihood and the future. Effort belongs in action, not in worry; rely on G-d's mercy (IK 4274, 3275). Don't let worries about the future disturb your learning or overshadow present blessings (IK 2864, 6587, 6869, 8488), and don't worry about decisions before their time (IK 7044). Seek work that allows peace of mind (IK 7158). See also: Livelihood – Worry about money and trusting in G-d.
Practical steps the Rebbe often advised: checking tefillin and mezuzos, daily tzedakah (especially before prayer), daily Tehillim, reciting Shema before sleep, and saying and learning Psalm 23 (IK 3871, 6481, 7928, 8757, 9571). For sleeplessness, prepare properly before sleep (IK 4302, 7928). Share what is weighing on you with a trusted friend and seek guidance from G-d-fearing people (IK 3536).
Choose joy. Discouragement and sadness come from the yetzer hara, not from the facts; serving G-d with joy lifts them (IK 1042, 3079, 3295). Pesach is a time of freedom from all worries (IK 10746).