Losers believe in luck; lucky people believe in themselves. Do you agree with that? I very often doubt my actions and am not sure of myself. How can you convince yourself that I'm not more stupid than others?

There are two options: either go to a psychologist or raise your self-esteem by passing an IQ test https://iq-global-test.com/ Even if your intelligence level turns out to be average, at least for you it will be a reason for self-development. And as you develop, you will definitely honor yourself with self-confidence.

Frequent mood swings can be related to hormones. Have you consulted a doctor? If this is all right, then I would recommend consulting a psychologist. An unstable emotional state may be associated with some unpleasant event. Australia Counselling has an excellent team of professionals at https://www.australiacounselling.com.au/. You can choose a consultant near your home or work. It is very convenient.