Doctor - there's also medicine to become a Psychiatrist, you can specialise in children and young adults mental health, known as CAMHS, or in substance abuse.
Peer support - working within a charity such as Mind, but that normally requires a personal experience of mental health problems or substance abuse problems (depending on what the group is).
Counselling - self explanatory, and seems to fit what you're saying? Quite a competitive career I believe
Clinical psychologist - different to medicine, often requires a PHD, can deliver CBT and things, often present on the wards in Psychiatric units, and are out-patient based too
Social worker - can work mainly in children's and families
Carer - helping deliver the practical aspects of day to day life for someone living with mental health problems
TBH there's loads of supportive roles within the mental health system - such as working at houses that allow people to step down from a Psychiatric inpatient unit, advocating for people with mental health problems (advocacy worker), being a youth worker, being an appropriate adult, a care co-ordinator, etc
Maybe volunteer with some mental health specific charities like Mind, and see what sort of job roles people have. You could also try and get work experience shadowing a Psychiatrist or mental health nurse, but this might be very difficult to arrange.