Hypnosis' effectiveness is directly related to how you can allude yourself to believe a certain suggestion. But don't think that renders it invalid.
I took psychoanalysis hypnosis sessions on a therapeutic level and they greatly improved my life. You have to really focus during the "induction phase" which is where the host will help you visualize certain stimuli that's designed to "put you under". Common examples include "walking into the ocean", "going deeper into a house", "walking through the woods", or even spreading out your 'focus' to focus on several different body parts at once, breathing, toe, shoulders, nose, etc. The human brain isn't designed to process that amount of input... so when "hyper-focusing" on your body occurs, it's a form of meditation that will kind of yank away layers of higher processing so that the hypnotic host can directly interact with the cognitive "switchboard" of your brain.
If you can motivate yourself to do simple things such as lose weight or believe in yourself, hypnosis, honestly, falls under the same principle. It shares ghastly similarities to the movie "inception". An indicator of a relatively successful hypnosis session involves a /slight/ lean towards the intended effects of the track, plus, unyielding relaxation and a vague memory of the process after waking. I know I've been hypnotized rather successfully when I can't remember what I did for the past hour.
Remember, you have to focus during the induction phase, repeat/layer the hypnotic sessions and know that hypnosis has truth in theory. It all boils down to how well you can convince yourself to do something.