What can i do to forget about old addiction/obsession?

Theoretically: You never forget an old addiction or obsession. You can use behavior modification techniques and/or hypnosis to reduce their impact on you as much as possible. From aa and research we know that once you are addicted to a substance you are addicted for life, even if you are sober for years. So see a psychologist and get help reducing the cravings as much as possible.