What is the difference between ssi / vesid / & workers comp ?

Ok I'm 18 and I have ADHD and was diagnosed with bipolar disorder I'm currently on stable meds and see a doctor ... I just would like to know what the differences are in the 3 systems of how people with disabilitys get help with funding ??? I went to my local ssi office and they rejected my request for monthly help so I think of it as no big deal ... Well I have a friend who is the same as me and he's gotten 2 settlements from vesid for 15000 dollars each ... I was wondering what would happan if I were to lose my job becuase the ssi office told me that " I havnt worked enough " I'm still young but I'm in such a struggle and need help I live at home with my 56 year old disabled mother and I make 8 bucks an hour at a local fast food joint .... There has to be a way I just don't want to contact vesid or workers comp and sound like an idiot becuase I'm not sure of what to tell them ... In my case it would be considered a mental disability and it's been diagnosed since I was 12 and what would be my options am I allowed to settle or get monthly help ??? I don't get how it works I'm still young bear with me ..