Why do I feel so inferior and worthless and useless all the time?

You could possibly be suffering from Depression and Anxiety, its quite common to feel this way when you are feeling down, sometimes you may just want to separate yourself from people or to not want to do anything. To make yourself feel better but it actually makes it worse, avoiding the problems rather than deal with them. They are all linked together in a vicious circle. You have to challenge the negative thoughts and get active, do stuff about them to get rid of them! (Personal experience!)

It's possible you had a bad experience in your childhood. Maybe you were bullied, or you may simply have depression. It would probably be a good idea to see a psychologist.

It can be down to how you view yourself. If your thoughts towards yourself are negative ones such as "I'm not as good as other people" and "I'm useless" it can lead you to believing them which can give you low self esteem and a negative view of yourself. These thoughts are not true because thoughts are different to facts, but thinking them for long enough can make them seem like facts, making you feel like you actually are inferior and useless. These thoughts can be changed, which can lead to a more positive view and self esteem

Because everything i do is a contradiction of something I said, or vice versa. Because my brain's so messy I've given up trying to understand myself.

Because you compared yourself to others, Value in one's worth or usefulness is infinite if you don't

this could be because of depression you may want to see a doctor it could also be because you are thinking in all or nothing terms

This can be a symptom of many mental illnesses and it can be a result of abusive and mistreatment in your life and with enough help you can find a way to get past this side affect.

Well, this might be related to your thought patterns... the way you see things makes you inferior, now you can support yourself by changing this... you can start to appreciate yourself for the simple achievements in your life and slowly you gain love for yourself.

usually when things are not in your favor, or if something hasn't quite gone your way can sometimes make you feel worthless and inferior.

I'm not sure, only you know that. There must be something going on to make you feel like you are that - but I am sure you aren't.