Can I adjust myself and do I really need a chiropractor?

No, you cannot adjust yourself.  Manipulating vertebrae that are already too moveable doesn’t do a thing for your nerve pressure and in fact may do damage to the joints and the spinal curves.  However the vertebrae that are fixated or jammed (hypomobile) aren’t affected by “self-adjustment” demonstrations.  Unfortunately, these are the ones that are in need of the adjustment.  So yes, it is possible to “crack” yourself but as a rule it is not possible to give yourself an adjustment.