You're obviously experiencing some very intense emotions right now, but you're not out of your own control and you're not going to commit a crime. It's important to understand that while your anger is manifesting in a very specific way, the problem doesn't actually have anything to do with Emma Watson specifically. It's more likely that you're feeling a lot of pressure and anxiety that relates to some of the aspects of Emma Watson that you mentioned, such as intelligence, physical appearance, and social popularity. If you're currently unmedicated for your ADHD then that may be contributing to the racing thoughts or more extreme moods that you've been experiencing.

I would recommend that you discuss how you've been feeling with your psychiatrist, and look into some cognitive behavioural therapy techniques that focus on restructuring the type of negative thoughts that you've been having and using breathing exercises to reduce your anxiety. You mentioned cutting/scratching your arm during the height of the episode, which is often a behaviour that individuals engage in to release the build up of anger or mania that they're experiencing, as well as providing adequate distraction through self-controlled physical pain. Although self harm may seem like an effective method of releasing one's emotions and ending an episode of rage or anxiety, studies have shown that on average these episodes are the same duration (10-15 minutes at most) whether the individual self harms or not.

If your parents are supportive, you could consider telling them that you would rather them not talk about celebrity gossip at all, either directly with you or around you for a period of time. It doesn't sound like it is an integral part of your life, so that shouldn't be an issue. Also try to remember that the Emma Watson that you see through the media is not even really an accurate reflection of one single person. The celebrities that the public know through the media are calculatedly composed images that are created and edited by a team of public relations professionals. I'm not trying to say that Emma Watson wouldn't be attractive or intelligent if she wasn't a celebrity, but celebrities are more like projects that many people contribute work to, as opposed to individual people who are existing.

Good luck with all of this, and sorry for rambling on for so long in my response.