When you are hypnotised and it works does it stay that way for good or does it go away?

Depends: When hypnosis works, it can be long lasting and for others it is the beginning of the changes and needs further work. The positive work that has happened is great and i would let that help you lead a better and healthier life. If you need it again, go back until you feel comfortable again.

Hypnosis permanence: This differs, depending on the complexity of the problem. In hypnotic trance, you're being given suggestions for change. A superficial change that sounds good to conscious mind might "last" after only 1 session. A more complex problem usually needs repeated suggestions for at least 21 days to develop new neural pathways. That's why hypnotists often encourage listening to recordings of sessions.