Well, what's the following?
Family - would be a big one. It's a person's social environment in which they are the most influenced. This environment is where an individual learns values, develops his personality and opinions as well as attitudes the world and on themselves.
Peers - When you are a teen to young adult, you begin to notice differences in your peers and maybe compare them to yourself.
Experience - If you're had successes, you'll tend to feel good about yourself. If you've experienced lots of failures, you'll feel less confident about yourself.
Media and Advertising - The people in advertisements can tend to be seen as the standard of beauty, and/or size.
Trauma - Experiencing types of trauma in life could cause a person to develop a negative self-concept.
Health - Some may have health problems that require them to need the assistance of others. This could cause them to feel bad about themselves for having to be a burden or feelings of inadequacy.
Economic Status - Individuals in a lower economic status may feel low about their situation. If they aren't able to afford the newest shoes and clothing, or have the nicest things, they may feel somewhat invaluable or worthless. Ones from a higher economic status may feel good about themselves. They have nice clothing, nice vehicles, nice homes, and are probably popular so that attracts compliments and makes them feel relevant.
This is heavily disputed, and honestly, it varies on a person-by-person basis. Some people are visual learners (so it's effective for them to use videos and pictures to help them study), some prefer being hands-on, etc...
Generally though, I'd say the most effective method that works for most people is through visceralisation. This is the process of associating concepts, words. phrases etc. with your different senses (sound, touch, images...). You can read up more on it by searching up "visceralisation for studying".
