The Rebbe's guidance for those who feel inadequate, unworthy or unsuccessful:
The feeling is not the truth. Doubts about one's abilities and feelings of inadequacy are tactics of the yetzer hara, not a true assessment (IK 2219, 3623). Feelings of failure are unfounded; trust in G-d and dedication to Torah and mitzvos bring real success (IK 5660). Every Jewish soul has immense worth (IK 10615), and each person has a unique Divine mission; true self-confidence comes from recognizing it (IK 10529).
Look outward, not inward. Instead of dwelling on one's own perceived shortcomings, give yourself to helping others with warmth and love (IK 1931). Teaching Torah raises a person's worth rather than lowering it (IK 2219). Everyone can deeply influence others through knowledge and personal example (IK 10452). When feeling unsuccessful in communal work, renew your motivation and look again at your purpose (IK 4140). Trust that G-d provides what you need even when you feel undeserving (IK 3468).
Keep going. Persevere in teaching or work despite doubts about yourself, finding a supportive environment and consulting those who know you (IK 10274). Overcome shame over the past by turning it into present strength (IK 4431). True leadership rests on self-confidence rooted in being trustworthy (IK 7440).
Specific challenges. For speech difficulties and stuttering, combine professional help with Torah study and building self-confidence (IK 7173, 7299). A child's self-confidence can be built by giving him responsibility and chances to help others (IK 5494).
See also: Torah study – Motivation, diligence and study partners; Bitachon – Joy and overcoming sadness.