Emotional Distress:
Death of a loved one
Surviving childhood abuse
Being abandoned by a parent
Betrayal by a romantic partner
Being bullied/excluded
Long-term illness/disability
Surviving a natural disaster
Experiencing discrimination
Combat trauma from war
Witnessing domestic violence
Rejection from a dream career
Surviving sexual assault
Growing up in poverty
Being wrongly imprisoned
Crippling social anxiety
Major sports injury ending career
Losing your family home
Wrong mental illness diagnosis
Sudden death of a spouse/child
Betrayal by family/friend
Failing at your life's passion
Surviving a terrorist attack
Causing a fatal accident
Attempted suicide
Life-changing disability
Surviving a kidnapping
Infertility issues
Narcissistic parent upbringing
Religious disillusionment
Public humiliation from scandal

Here is a list of potential internal, mental, and emotional conflicts that could create complexity for a fictional character:

Self-doubt/lack of confidence
Perfectionism/fear of failure
Guilt over past mistakes/actions
Self-destructive behaviors
Addiction/substance abuse
Anger issues/explosive temper
Anxiety/panic attacks
PTSD/trauma flashbacks
Depression/ suicidal thoughts
Eating disorder/body image issues
Insecurity/low self-esteem
Shame/embarrassment
Grief/inability to move on
Jealousy/envy of others' success
Prejudice/close-minded thinking
Chronic indecision/fear of commitment
Imposter syndrome
Resentment toward a parent/caregiver
Feeling undeserving of love/relationships
Manipulative/dishonest tendencies
Obsessive thought patterns
Difficulty expressing emotions
Avoidant attachment style
martyr complex
excessive need to be liked/approved of
religious/spiritual disillusionment

medical disorder/condition on its own line:

Generalized anxiety disorder
Social anxiety
Phobias
OCD
PTSD
Major depressive disorder
Bipolar disorder
Alcoholism
Drug addiction
Opioid addiction
Gambling addiction
Anorexia nervosa
Bulimia nervosa
Type 1 diabetes
Type 2 diabetes
Asthma
Systemic lupus erythematosus
Rheumatoid arthritis
Multiple sclerosis
Fibromyalgia
Migraines
Chronic back pain
Insomnia
Sleep apnea
Narcolepsy
ADHD
Hypothyroidism
Hashimoto's thyroiditis
GERD
Acid reflux
Irritable bowel syndrome
Food allergies to nuts
Food allergies to seafood
Food allergies to lactose
High blood pressure
Hypertension
Heart disease
Heart failure
Arrythmias
Stroke
Transient ischemic attack (TIA)
Epilepsy
Tonic-clonic seizures
Hearing loss
Tinnitus
Cataracts
Macular degeneration
Glaucoma
Eczema
Psoriasis
Osteoarthritis
Rheumatoid arthritis
Gout
Chronic kidney disease
Kidney stones
Anemia
Iron deficiency
B12 deficiency
Urinary incontinence