How do you define self-hypnosis? It's actually a state many of us achieve at times without even being aware of it. Daydreams, meditation, the trance-like state we can experience when we become totally engrossed in a book or project, all of these are very similar brain states.
The key difference between these and deliberate self-hypnosis are that the latter involves specific intent, and the use of suggestions targeted towards a goal - thus self-hypnosis is always done deliberately and with a purpose.
Self-hypnosis differs from regular hypnosis only in that there is no third party (hypnotherapist) involved. In every other way they are identical. Thus both can be defined as:
A state of heightened focus and awareness in which?
Critical faculties are suppressed such that?
Suggestions are much more likely to be acted upon than in normal conditions
Indeed, at a certain level, some therapists believe that all hypnosis is in fact self-hypnosis, as the patient always remains in control.
Once you learn this skill for yourself, however, you are freed from the need of consulting a therapist - you have a powerful tool you can use for yourself, as frequently as you wish.
Learning this skill for yourself is ultimately much more beneficial than paying for a handful of sessions with a third party. As the old saying goes, "give a man a fish and he'll eat for a day - teach him how to fish and he will feed himself for the rest of his life."