The devil can plant thoughts in your mind. Diabolical obsession is the first level of temptation. It deals with spiritual attacks on the mind. If you use hypnosis, you are giving access of your mind to demons. I'm not going to tell you that the type of hypnosis you're using is "okay" or it's not a sin because you didn't go into a trance. It's all bad. Even if this didn't have a spiritual element to it, it would still be wrong to let someone hijack your consciousness and mess with it. If you knowingly consent to these kinds of things even after you've read all kinds of warnings not to, you are committing sin. I think the Pope and Cardinal Sarah have also said not to use hypnosis because it is allowing someone to manipulate your mind and soul. Just stay away from it. It's not so important to "feel fat" that you risk mind control, damnation, and loss of autonomy over it.